A Portable Pit Latrine Emptying Machine The
eVac DRAFT Angus McBride â€“ May 2012
Summary In developing countries around the world many millions of people have been encouraged to use pit latrines in an attempt to end open defaecation. Little thought has previously been given as to what to do when these fill up. Due to space or cost constraints it often makes more sense to empty the pits rather than build new pit latrines. Pit latrines are typically emptied either by vacuum or manually. The former is generally carried out with large vacuum tankers (typically five to ten cubic metres in capacity) whilst the latter is done with buckets and spades and is a hazardous and unpleasant occupation. Large vacuum tankers are in many cases not an option because of their cost and the difficulty in getting them close enough to the latrine site. The eVac has been developed as a cheaper and more portable alternative to a vacuum tanker. Using a small vacuum pump mounted on a trolley, the eVac creates a vacuum in a container which in turn sucks up the sludge from the latrine. The eVac uses off-the-shelf parts, is light and is simple to use. By following a simple operating procedure exposure to sludge can be minimised. The eVac can only be used for pits which contain a fairly liquid sludge. If the sludge is too dry, compacted or if it contains a lot of rubbish manual emptying is advised. Manufacturing the eVac will cost between 18,000 and 22,000 ZAR (US $2,200 â€“ $2,700), which makes it easily affordable in the scope of a pit latrine emptying project.
Contents Preface............................................................................................................... 5 1. Introduction ................................................................................................... 7 Pit Latrine Emptying ............................................................................................................... 7 History of the eVac............................................................................................................... 12 Concept ................................................................................................................................ 14 Emptying Principles .............................................................................................................. 15
2. Anatomy of the eVac.................................................................................... 19 Overview .............................................................................................................................. 19 Trolley .................................................................................................................................. 21 Motor ................................................................................................................................... 26 Pump .................................................................................................................................... 28 Pump Protection .................................................................................................................. 34 Containers ............................................................................................................................ 38 Air Hoses and Fittings .......................................................................................................... 46 The Air and Water Lance...................................................................................................... 48 Sludge Hose & Fittings ......................................................................................................... 50 Instrumentation and Pressure Control ................................................................................ 53
3. Operating with the eVac .............................................................................. 57 Aims when operating the eVac ............................................................................................ 57 Personal Protective Equipment ........................................................................................... 59 Equipment required ............................................................................................................. 62 Emptying Procedure............................................................................................................. 64
4. Costs ............................................................................................................ 65 Construction Cost................................................................................................................. 65 Operating Cost ..................................................................................................................... 66
5. Bibliography ................................................................................................. 67 Appendix I: Cost Breakdown .................................................................................................... 69 Appendix II: eVac Field Test 2 .................................................................................................. 70 Appendix III: eVac Field Tests 3+4 ........................................................................................... 74 Appendix IV: eVac Field Tests 5+6 ........................................................................................... 79 3
Preface This report is written for those interested in replicating the eVac for use in a pit latrine emptying project or for further development, and for those interested in developing their own method of emptying pit latrines. The eVac has been developed in KwaZulu-Natal, South Africa, using parts that can be sourced from here and for the conditions found here. In other areas spare parts availability and the types of pit latrines will differ. There may also be different access problems, and the typical contents of a pit will vary. Any person replicating the eVac in other parts of the world must first be aware of what conditions the eVac will be operated and maintained in. I have endeavoured to include observations throughout that may be useful in this. The eVac has been developed as part of a project undertaken by Partners in Development in collaboration with Engineers Without Borders UK. The project has been funded by Water for People, and is the continuation of a project funded by the South African Water Research Commission supported by Irish Aid. This and related projects have developed a large body of knowledge regarding pit latrine emptying and disposal. Useful references to these can be found in the bibliography. Angus McBride Pietermaritzburg, May 2012
1. Introduction Pit Latrine Emptying In developing countries around the world many millions of people have been encouraged to use pit latrines in an attempt to end open defaecation, which is a major cause of disease. Ventilated Improved Pit (VIP) latrines are the tool of choice for most governments and NGOs in Africa (Figure 2), whilst holes in the ground with a couple of planks on top are often used when poor households are left to build their own latrines (Figure 1). A problem often not considered is what to do when these latrines become full. In rural areas where cheap structures have been built by home owners a new latrine can easily be constructed, but in urban areas where space is limited this will not generally be possible. Additionally, the construction of safe, desirable, permanent structures can only be done at considerable cost, and so emptying existing pit latrines is likely to then be a cheaper option. Using a vacuum tanker to empty pit latrines is a good solution where it is possible. Unfortunately in some informal settlements and rural areas access to pit latrines is not always possible for these large vehicles. Additionally, they can struggle to empty some drier pit latrines without the addition of a large amount of water and the large amount of rubbish found in some pits can block the suction pipes. They also have a high capital cost. In many places pit latrines are therefore emptied by hand. This can be fast and is very low cost, but it does put the health of the emptiers at risk. The eVac is the latest in a series of attempts to find a machine which can occupy the middle ground between hand emptying and using a vacuum tanker. In 1990 the Dutch NGO WASTE developed the Manual Pit-latrine Emptying Technology (MAPET), a hand powered vacuum pump connected to a mobile 200 litre drum. This was tested in Dar es Salaam with some success but was not continued. Steve Sugden developed the Gulper, a very simple manual pit emptying device, which has since been developed and used by Oxfam (Figure 3). The Nibbler and Gobbler were chain-based hand driven devices which had limited success. The Pit Screw Auger is a motor-driven device which uses a screw elevator to remove sludge but has struggled with the rubbish found in pit latrines (Figure 4).
The Vacutug (Figure 5), Dung Beetle (Figure 6) and Maquineta are three smaller vacuum systems that have been developed. More information on these can be found in the report “Tackling the Challenge of Full Pits, Volume 3: The Development of Pit Emptying Technologies” by Partners in Development and the Pollution Research Group, which is to be published as one of the outputs for the South African Water Research Commission project K5/1745.
Figure 1 – A typical home built pit latrine
Figure 2 â€“ Latrines built by South African municipalities
Figure 3 (left) â€“ The Gulper, now being used by Oxfam Figure 4 (below) - The Pit Sludge Auger during testing
Figure 5 - The Vacutug, developed by UN Habitat
Figure 6 - The Dung Beetle
History of the eVac The eVac is a continuation in the development of the Nanovac, a simple piston pump which created a vacuum in a container into which sludge was sucked (Figure 7). This avoided the need for sludge to travel through the pump. The Nanovac was developed to overcome some of the challenges the Vacutug faced; principally that it was not easily transportable. Revolving around the use of large diameter piston pumps to suck and blow air the design mirrored that of the MAPET system in Dar es Salem. The Nanovac aimed to be low cost, compact, easy to manoeuvre, easy to maintain and simple enough to be replicated almost anywhere in the world using local parts. Despite some success, the simple piston pump proved not to be powerful enough to empty real pit latrines, and was beset with ongoing maintenance problems. The piston pump was substituted with a commercially available vacuum pump and this is what has become known as the eVac (Electric-VACuum). A variety of vacuum tanks have been tested during the development of the Nanovac and eVac. Initially the tanks used were large (100 and 180 litres), but these were difficult to transport and took a fairly long time to develop a vacuum in. The current model uses small tanks which are easy to handle and allow for effective use of the plug and gulp suction system.
Figure 7 – The Nanovac – the eVac’s predecessor – at different stages of development
Concept The eVac must meet several criteria. It must: Be simple to use, Be easily portable over rough ground requiring no more than two people, Be affordable for use in a pit latrine emptying project, Not need a specialised vehicle to transport it, Be capable of emptying a wide variety of pit latrines, including those that have some rubbish in them, Limit, or ideally prevent, emptiersâ€™ and the surrounding areaâ€™s exposure to pit latrine sludge, Not require specialist skills to manufacture or maintain that cannot be found in a developing country, Use parts that can be sourced and maintained locally.
Emptying Principles While the Ventilated Improved Pit (VIP) latrine principle is by far the most common type of pit latrine a number of different designs exist. One thing that most have in common, however, is that they were designed with little thought for how the pit will eventually be emptied. Pit latrines designed with in-built pipes for fluidising or sucking the sludge have not yet left the drawing board and it is uncommon for latrines even to have easy to remove slabs covering the pit (Figure 8). The superstructure of some latrines makes the pedestal easy to access (Figure 9) whilst others make for cramped working environments (Figure 10). Some pits are situated completely below the latrine superstructure, some are completely offset from the superstructure and some are in-between. The sludge inside pit latrines varies almost as much as the design of the pit latrine itself. Often the sludge is very dry, with any liquid draining into the surrounding soil, but equally often the sludge is wet because the pit extends below the water table or rainwater is able to enter. There may or may not be rubbish in the pit, and this rubbish can range from the occasional item to full bin bags or rags. Furthermore, the sludge found at the top of a pit will not be the same as that found at the bottom: both degradation through microbial action and consolidation are likely to have made the sludge at the bottom a lot more dense than that at the top. Any vacuum based system will find it easier to pump more watery sludge. Drier sludge is challenging because of its higher density and it becomes more difficult to form an air-tight seal, allowing air to leak through the sludge and into the suction pipe.
Figure 8 (top) – A very easy to access latrine- most are not like this Figure 9 (bottom left) – A more open working environment Figure 10 (bottom right) – A very cramped working environment 16
Manus Coffey lists various approaches that suction systems can use to deal with sludge (Figure 11): High vacuum / low airflow approach: the hose is submerged deep under the sludge, and atmospheric pressure (Pa) acting on the surface forces the sludge along the hose into the holding vacuum tank (Py). With a low vacuum / high airflow approach: air is constantly dragged through the system and particles of sludge are suspended in the very high air stream and drawn along the hose into the holding tank, as with a domestic vacuum cleaner. Air bleed system: a pipe is inserted into the sludge. With a combination of high vacuum and medium airflow, the atmospheric pressure forces air down the air bleed pipe and thus maintains the airflow necessary for the sludge particles to be suctioned. Plug and gulp: a combination of high vacuum and medium airflow is used, with an air drag effect obtained by raising and lowering the hose inlet in and out of the sludge.
Figure 11 - (Clockwise from top left): High Vacuum/Low airflow approach, Low vacuum, high airflow, Air bleed system, Plug & Gulp (all from Manus Coffey)
The eVac uses the plug and gulp system, which allows it to deal more easily with the high densities of sludge in pit latrines. This and the desire to make the eVac easy to move mean that the containers used are small. The containers must then be emptied either into a disposal pit or into a larger container for transport. There are two distinct approaches that have been developed for emptying the containers: 1. Suck Only Several containers share an easily removable lid which has the sludge and air hoses attached. Whilst one container is being filled other containers are being emptied by carrying them and tipping them out. The tank only needs to withstand vacuum pressures and the vacuum causes the lid to self-seal 2. Suck & Blow Once the container is full the contents are blown out through a separate pipe. Only one container is used and the lid is bolted on to resist the positive pressures experienced.
2. Anatomy of the eVac Overview
Extra Interchangeable Lid
Pump Unit Sludge Hose Sludge Container
Figure 12 - The eVac arrangement when being used in the suck-only setup 19
Suck & Blow Container
Figure 13 - The eVac arrangement when being used in a suck and blow setup 20
Trolley A custom fabricated steel trolley forms the chassis of the eVac (Figure 14, Figure 15, Figure 16, Figure 17). The motor and pump are mounted to the bottom of the trolley and are connected by a belt drive. The oil supply for the pump is mounted above it, as are the vacuum relief valve and the moisture trap. Despite the trolley unit weighing a total of 63kg the trolley is manoeuvrable, is capable of being moved easily across rough ground and can be lifted onto a vehicle by two people. The trolley is also stable when standing upright because the pump and motor are mounted low down on the trolley. It might seem convenient to have spaces on the trolley to attach the hoses for storage, but this would risk contaminating the machine if the hoses got dirty. It would also make the machine much more unwieldy to start and move.
Vacuum Hose Connection Vacuum Gauge Moisture Trap Vacuum Relief Valve Oil Reservoir
Motor Control Box Pressure Valve
Pressure Hose Connection Pressure Gauge Belt Gaurd
Figure 14 - Top view of the trolley
Vacuum Gauge Vacuum Release Valve Oil Reservoir Moisture Trap
Pressure Gauge Pressure Valve
Pressure Connection Motor Box
Figure 15 - Front view of the trolley
Relief Vacuum Gauge
Vacuum Camlock connection Pressure Valve
Figure 16 â€“ Rear View of eVac
Vacuum Relief Valve Oil reservoir Pressure Gauge Vacuum Camlock connection
Pressure Relief Valve Pump
Moisture Trap Pressure Camlock connection Belt Guard
Figure 17 - Side view of the trolley
Motor The eVac is powered by a 1.5 kW (2HP) single phase electric motor with an attached control box (Figure 18). The control box is a weak point in the machine as it is not designed for the rough use the eVac receives and the plastic around the screws that hold the lid on have cracked. If the eVac were to go into production a more robust solution must be found: possibly by welding on some metal bars around the control box as a roll cage or by using a steel control box. The long length of power cable coming from the control box was difficult to store satisfactorily on the trolley and at one point ripped right out of the control box when it got caught on something. Inspired by garden machinery, there is now only a very short length of cable coming from the control box and an extension cords is necessary. 2.5mm cable must conservatively be used, given the 1.5kW motor power. Being able to transport the cable separately is more convenient. The motor requires a standard 230V power source and for this a generator can be used if a nearby power source is not available. Using a petrol engine in place of the motor would alleviate the requirement for an electric power source but would make the eVac much less manageable and more susceptible to damage. The pump needs to turn at a fairly constant rate which means that a governor would need to be used.
Figure 18 - The 1.5 kW motor that powers the eVac, with control box on top
Pump Specialised sludge pumps exist but are not generally suitable for use in pit latrines. No pump that could be easily sourced and serviced in a developing country could pump such a varied and difficult substance as pit latrine sludge. A vacuum pump is therefore used to create a vacuum in a container, so that the pump itself only ever pumps air. There are many different types of vacuum pumps. Positive displacement vacuum pumps are the only category of vacuum pump suitable for this application, and of these rotary vane pumps (Figure 19), diaphragm pumps (Figure 20) and liquid ring pumps (Figure 21) are the most common. Liquid ring pumps are only suitable for industrial applications as they need a supply of fresh water to keep them cool and whilst a diaphragm pump could be used it is not as simple to service as a vane pump, hence the eVac uses a vane pump. The following is a useful introduction to vane pumps by Daniel Pascoe, General Manager of Vacuforce in America: “The rotary vane type vacuum pump is one of the most popular vacuum pumps in use today. Manufactured by many companies throughout the world, this type of vacuum pump is commonplace in most industrial vacuum environments including packaging, thermo forming, hospital systems, and vacuum-handling applications. “The simple construction of vane pumps makes them rather inexpensive to manufacture and they offer relatively good value to the vacuum user. There are two basic types: (1) lubricated and (2) non-lubricated, which is sometimes referred to as “oil-free.” Both of these pumps operate using the same basic principles but are designed for different applications. And this is important to understand and appreciate as the wrong selection could lead to malfunction of the equipment. “The obvious reason for selecting an oil-lubricated rotary vane vacuum pump is because the user requires a higher vacuum level than that offered by an oil-free version. Oil-free vacuum pumps are quite capable of generating a vacuum level of 0.9 bar (90% vacuum). Of course, as with any vacuum system, the application has to be sealed or the pump “dead headed” to achieve this maximum vacuum.
Figure 19 – Schematic of a vane pump
Figure 20 – Schematic of a diaphragm pump
Figure 21 – Schematic of a liquid ring pump
“… Basically, an oil-free unit is less expensive and easier to maintain…. “There are two fundamental problems when a lubricated pump is used at a low vacuum such as <0.7 bar. The oiled pump system requires a vacuum to be generated within the pump to pull the oil through to lubricate the vanes. There is not a mechanical oil pump doing this. Consequently, if the vacuum level is not high enough, poor lubrication occurs. Furthermore, and one that is often more apparent to the user, is that if an oiled pump is run at a low vacuum, the airflow through the pump is excessive and oil carryover can occur. “One of the benefits of a rotary vane pump is the simplicity of construction and therefore, maintenance. There are, of course, obvious periodic maintenance items such as seals, bearings, and other rotary machinery failure, but this is a pump breakdown or overhaul requirement. The standard service replacement is the vanes, which, depending on the manufacturer, are normally manufactured from aluminium or in the majority of cases, carbon. Most vanes will last in excess of 10,000 hours of service and in some cases, in excess of 20,000 hours. This is dependent on the way in which the pump is used, the vacuum levels the pump experiences, and the actual application. For example, a pump running a low vacuum will experience a fair amount of vane wear as the vanes are continuously under load and experience a lot of force put upon them by the amount of air they are continuously moving through the pump. A sliding vane vacuum pump will last longer when it’s continuously pulling a high vacuum. “Another advantage of lubricated vane pumps is their ability to handle a certain amount of liquid vapour (steam) from the application, normally non-corrosive, such as water. “The largest rotary vane pumps have a flow capacity in excess of 1,700 m3/hour. The pump size selection is based on the speed in which the air in the system needs to be evacuated. A large vacuum pump and a small vacuum pump of the same design offer the same vacuum level. It’s simply how quickly it can achieve it that determines model choice.”
Figure 22 - The inside chamber of the eVac's vane pump
Figure 23 - The vanes inside the vane pump 31
The eVac uses a small oil lubricated vane pump sourced from a dairy equipment supplier. It can produce an air flow of 300â„“/min at 0.5 bar vacuum (Figure 22, Figure 23, Figure 24). The eVac uses the exhaust of this pump to provide positive pressure. Although useful there are problems with this: With low vacuum pressures there is significant oil carryover into the exhaust air flow The air is very hot, possibly because it contains the oil particles The vane pump uses a large amount of oil, probably because of the low vacuum pressures it runs at most of the time. Additionally, the oil container (Figure 25) tends to leak when in transit, if not kept upright, and is susceptible to the inevitable knocks it receives. It may be desirable in future to have an oil-free vane pump. The pump has been serviced once following repeated sludge entry before the development of adequate pump protection.
Figure 24 (above) - The vane pump in place on the eVac Figure 25 (left) - The oil reservoir that supplies the pump
Pump Protection A vane pump is designed to pump air only. Any sludge, solids, or even significant amounts of liquid entering the pump are likely to damage the moving parts. This must be prevented in a fail safe way which does not rely on the operator doing anything. Two measures are used to prevent any sludge entering the pump.
Stage One: Primary Float Valve The primary float valve is attached to the lid of the sludge collection container. It is simply a float contained in a short length of PVC pipe over the vacuum outlet (Figure 26, Figure 27). When sludge in the tank reaches the height of the float the float rises and blocks the vacuum line before any sludge can enter it. A squash ball was chosen as a float after tests with various types of ball. It works well because it floats but is heavy enough not to be sucked upwards by the air stream. Furthermore it is elastic and ductile enough to block the vacuum line well. The primary float valve is not infallible, however, and splashing of the liquid in the tank can allow some sludge to escape past the float valve before it closes. A second stage of protection is therefore necessary.
Figure 26 â€“ The Primary Float Valve
Screw holds float inside pipe
Figure 27 - Diagram of primary float valve open (top) and closed (bottom)
Stage Two: Moisture Trap The moisture trap is located on the trolley just before the pump. Its function is to catch any liquid that manages to get through the primary float valve and to act as a backup safety measure should the float valve fail. A thorough search was made for locally available moisture traps, but none were available of the type required. Most were designed for compressors, or similar, where only a very small amount of condensation has to be removed from air lines. They did not have the capacity to stop the large volume of liquid required for this moisture trap. As a result the eVac uses its own custom made moisture trap (Figure 28). The moisture trap has been made using a short (320mm) length of clear PVC pipe, 140mm in diameter, to which two end caps have been added. These fit tightly and do not need to be glued, although duct tape has been added to ensure a good airtight seal. Anything going into the pump must pass through this container, entering through a hole in the lid and leaving by an adjacent hole. Gravity will cause any liquid or sludge to sit at the bottom of the container, whilst only air should be sucked out of the top. Should the container fill with liquid, there is another float valve, similar to the primary float valve, which will block the suction line. This should only happen in the event of a complete failure of the primary float valve, as the container is self emptying: a brass one-way valve is attached to the bottom of the trap so that every time vacuum pressure is released any liquid in the trap will escape under gravity. The automatic emptying produces a new problem: any sludge in the moisture trap will simply be spilled onto the ground thus posing a contamination risk. A small container could be placed below it, but this is something that is very likely to be forgotten about.
Figure 28 - The moisture trap
Containers Initially, during the development of the Nanovac a steel drum was used as a vacuum tank to hold the sludge but this proved cumbersome. A Hippo Roller - an 80 litre water carrier designed to be rolled along the ground â€“ was tested as a vacuum tank with the idea that by sucking the pit sludge directly into a portable container which can simply be poured out, the need to suck and then blow from a vacuum tank would be eliminated. Under testing the roller tended to buckle slightly under a -0.25 bar vacuum. The ends were braced with a short length of pipe. This allowed a vacuum of over -0.3 bar to be reached. As an alternative vacuum chamber, a tipping tank was designed using a 48 kg domestic gas canister. The tipping design eliminated the need to rotate the tank, instead allowing it to simply be tipped from one orientation for filling to the other for emptying. This disadvantage of this tank was its weight, making it difficult to manoeuvre. A fibre glass vacuum vessel was then designed with a capacity of 100 litres (Figure 32). This was far lighter and had a steel frame made for it with wheels to make it easier to transport across rough ground. A larger fibreglass tank with 180 litres capacity was then fabricated. Due to its increased size this had to be reinforced with steel hoops to prevent it from imploding under negative pressure. Despite being relatively light these containers were still large and awkward to move. Their size also meant a long time was needed to develop a vacuum within them, which means that they were not suitable for the plug and gulp methodology, and it was because of this that smaller tanks were sought. Various small tanks were bought from local suppliers but no tank could be found which could withstand a vacuum (Figure 31). As a result of this the containers currently used have been custom manufactured. 47â„“ containers have been roto-moulded locally using Linear Low Density Polyethylene (LLDPE). They are 770mm high with a 310m diameter and have a wall thickness of 14mm. They weight 9.6kg each. An initial trial container with a wall thickness of 7mm was not sufficiently strong to resist the vacuum (Figure 30). Roto-moulding can be done in small quantities, which makes it suitable for this application. A steel mould is needed for the roto-moulding process and this is relatively expensive, pushing up the cost of containers if only a handful are to be made. The roto-moulders 38
where the eVacâ€™s containers were made had an existing mould of approximately the correct dimensions, and it is likely that other roto-moulders may have a similar mould as well. Handles have been added to the containers to make them easier to carry. These are short lengths of webbing held in place by very large diameter pipe clamps.
Figure 29 - The 47â„“ LLDPE vacuum container
Figure 30 (Above left) - A prototype roto-moulded container that wasnâ€™t strong enough failing under vacuum Figure 31 (Top right) - An off-the-shelf container failing to hold a vacuum Figure 32 (Bottom right) - A specially manufactured 100 litre fibreglass container
Suck Only Containers Currently three LLDPE containers are used, allowing one of the containers to be filled whilst the other two are being emptied. One interchangeable lid with the air and sludge lines attached is used which can be moved between the containers. Initially this was a modified off-the-shelf plastic lid (Figure 34) but this proved not to be strong enough so a steel lid is now used (Figure 33). The main part of the lid is made of 8mm steel plate with a thinner steel shim around the edge to make it sit well on top of a container. There is no attachment to the container, and the lid stays on by the force of the vacuum alone. A foam rubber strip has been used on the underside of the lid to create a good seal with the containers. The lid weighs 9.6kg: this could be made lighter by using a thinner steel plate, which at 8mm is quite conservative. A 3â€? steel elbow is attached to the lid, and the 3â€? sludge hose can be connected to this. The air line is connected to a 1â€? T piece attached to the lid. The other outlet from the T is connected to a ball valve which, when open, releases the vacuum in the container. Both of these connections were initially screwed on, but these tended to work loose in testing so are now welded in place. Attached to the inside of the lid beneath the air line is the primary float valve. This is attached by a long plastic nipple which screws in to the underside of the T.
Vacuum Release Valve
Sludge Hose Connection
Air Hose Connection
Primary Float Valve
Figure 33 â€“ The current lid design
Figure 34 â€“ An earlier lid design that failed because it was too weak
Suck & Blow Container When working with the eVac in the suck and blow configuration, only one container is used (Figure 35). Rather than have an interchangeable lid which is moved between containers the lid is bolted onto the container: only to be removed for maintenance or in exceptional circumstances. This allows the container to withstand positive pressure as well as a vacuum. The container requires two air hoses: one for vacuum and one for pressure. These hoses both pass through three-way valves before entering the container. On each of the valves one side is linked to the atmosphere and the other to a steel “T” which joins the container. The sludge inlet pipe and sludge outlet pipe both need valves that allow them to be closed so that sludge isn’t sucked or blown through the wrong pipe. Duckbill valves – one way valves which would not get blocked easily – would be ideal for this but could not be sourced locally, and as a result ball valves are used which must be opened and closed manually. The sludge inlet pipe is connected to this lid in a similar manner as for the suck only container, whilst the sludge outlet layflat pipe is connected through an attachment at the bottom of the container. The total weight of the container is 27kg, meaning that it can be carried by one person. As it does not need to be moved once in position this weight is not a problem. The dimensions of the container are not ideal for the number of pipes required. If the top of the container were larger there would be space to put both of the 3” pipes on the lid which would be a simpler design.
Figure 35 â€“ The suck and blow tank
Air Hoses and Fittings All of the hoses which carry air are of 1â€? nominal diameter. This diameter was chosen because it of the inlet and outlet of the pump, and has been maintained across all parts to simplify sourcing parts for manufacture and maintenance. The pipes used are flexible, and approximately 3m long. If necessary, several sections of pipe can be joined together (Figure 36). Camlock couplings are used throughout. These coupling are the most common type available in South Africa for the joining of flexible piping and irrigation piping so this was an obvious choice. Each coupling has a male part and female part, and the machine has been designed so that all hoses should have a male coupling on one end, and a female coupling on the other. The couplings on the machine and on the tanks are then set up so that it is (almost) impossible to connect a hose to the wrong place which has the benefit that pipes can be joined together more easily if a longer pipe is needed.
Figure 36 â€“ The air hoses (above) and detail of the cam lock couplings (below) 47
The Air and Water Lance The use of an air and water lance for loosening the fluidising dry sludge in the pit has been tested (Figure 37). By attaching the exhaust of the eVac to a 3m long 15mm diameter hollow stainless steel pole air could be forced into the sludge, the hope being that this would make the sludge easier to pump. Furthermore, the lance has been tested with a setup which allowed an air/water mix to be used. Using high pressure air to loosen material has a precedent, and a machine by Vac-Ex Ltd in the UK uses compressed air as a means for simple mess-free excavation (Figure 38). It does use a very large compressor, however. The air hose used is lighter than the Heliflex hose used for the other air hoses as it does not need to withstand vacuum pressures. It is also of a smaller diameter. A short section of 1â€? Heliflex hose is used as a casing to give it rigidity and stop it collapsing at the sudden bend where the hose connects with the steel pole. Tests thus far with the air lance have not been particularly successful, and very significant amounts of water needed to be added before any change was observed in the sludge consistency.
Figure 37 - The Air Lance in use on a trial pit
Figure 38 â€“ The Air-Vac by Vac-Ex Limited in the UK 49
Sludge Hose & Fittings Five metres of 3” flexible ‘heliflex’ hose is used to suck the sludge out of the pit (Figure 39). The hose connects to the container lid using a 3” camlock coupling (Figure 40). This can be difficult to operate when it gets dirty, but provides a good seal. Inserted into the end that goes into the pit is a plastic bushing to narrow the opening slightly, the aim being to prevent anything large enough that could cause a blockage from entering the pipe. A stainless steel pole was attached to this end using hose clamps so that the hose could be controlled whilst in the pit. This has subsequently been removed as it was not long enough to reach the bottom of the pit. A longer pole could not be used as it would not fit into the superstructure of a pit latrine. It would be possible to use a modular pole with sections that could be joined together. Separating these sections once finished however would force the operator to come into contact with the contaminated pole. The fewer parts that go into the pit latrine the lower the chance for contamination. The sludge hose previously left the plug and gulp tank vertically, but this left it liable to collapsing as it had to bend sharply towards the ground (Figure 41). A 90o elbow was added to the container lid so that the hose leaves horizontally (Figure 33), and this appears to have eliminated any problems with the hose collapsing. The suck and blow methodology requires another hose to be used for blowing the contents out of the plug and gulp tank. For this a layflat hose (Figure 42) has been used as it is cheap and easy to handle. This is also connected to the tank using a camlock coupling.
Figure 39 - The Sludge Hose
Figure 40 - Both ends of the sludge hose: a female camlock and a PVC bushing 51
Figure 41 - The collapsed sludge hose during testing
Figure 42 â€“ The layflat hose 52
Instrumentation and Pressure Control The prototype eVac that has been developed has a pressure gauge and a vacuum gauge (Figure 44). These are only for research purposes and would be unnecessary in a production model. It is possible to tell whether the pump is generating a significant vacuum by the noise it is making. The prototype also has a pressure relief (Figure 46) and vacuum relief (Figure 43) valve. These would also be unnecessary for a production model. The vacuum relief valve is unnecessary because all of the components can withstand the maximum vacuum pressure that the vane pump can create. The pressure relief valve is unnecessary as the tank leaks slightly at around 2 bar, meaning that the pressure cannot rise above this. All of these components are standard off-the-shelf components. An attempt was made to make a pressure relief valve using a design provided by Manus Coffey (Figure 45, Figure 47). This proved to be more difficult than anticipated as it was difficult to prevent the valve leaking air at low pressures. Leaving out these components from a production eVac will reduce the cost and make it more robust without sacrificing performance.
Figure 44 (above) â€“ The vacuum gauge Figure 43 (right) â€“ The vacuum relief valve
Figure 45 – The pressure relief valve based on Manus Coffey’s design
Figure 46 – The pressure relief valve Figure 47 (following page) – Manus Coffey’s pressure relief valve design 55
3. Operating the eVac Aims when operating the eVac It is important to keep in mind when emptying the pit why it is being done, and what the aims are as you are emptying. Typically these are (i) empty the latrine quickly, (ii) prevent contamination of the area, (iii) minimise the emptiersâ€™ exposure to sludge and (iv) minimise latrine repair required afterwards.
To empty the latrine quickly Keeping costs low is essential when emptying pit latrines and the more pits that can be emptied in a day the more efficient the operation will be. The time taken at each pit extends to more than just the time it takes the machine to empty a particular volume of sludge from the pit. The time needed to then dispose of this sludge into an adjacent pit or vehicle needs to be considered, and so does the setting up and putting away time.
To prevent contamination of the area One of the aims of using a pit latrine is to improve public health. If, when emptying the latrine, sludge is spilled anywhere then that aim is compromised and emptying the latrine becomes a lot less worthwhile. It is therefore crucial that no sludge is spilled and that, if it is, it is dealt with in a way which will prevent it contaminating anything or anyone.
To minimise the emptiersâ€™ exposure to sludge The pit emptiers will inevitably be exposed to some sludge, no matter what system is used, but the eVac has been developed to try to avoid exposure where at all possible. This principle is not universally accepted, Steve Sugden writes: â€œIn Bangalore, India, the unskilled manual pit emptiers are paid twice as much as unskilled labourers on a building site for work which is comparatively easy and only involves lifting a bucket 2m from the ground and tipping the contents into a large drum. With their main comparative advantage being the ability to handle shit and smile at the same time, pit emptiers can be keen to maximize the benefits these skills can bring by making the work appear more disgusting than it actually needs to be. 57
The pit emptying process in Bangalore is truly disgusting to watch and with a little ingenuity it could easily be made a lot more pleasant and cleaner. Ingenuity is not in short supply in India, but the emptiers know that if they improved the process by making it more hygienic they would have less of a negative impression on the householder and would argue for lower charges. It’s in the best interests of the emptiers to keep the process as dirty as possible. Mr Clean, who normally drives his vehicles around in a sharp suit says he occasionally walks about town dressed in his dirty overalls just to ‘put people off entering the business’. Seeing a bit of shit is alright and making too hygienic is bad for business…. It strikes me that hygiene is only every mentioned by Mzungus [white people], never the emptiers.”
To minimise repair required for the pit latrine Pit latrines come in all shapes and sizes, some are easy to access whilst others have no access except through the pedestal. Whilst it is always possible to knock a hole in the pit cover to access the latrine, this is something that should be avoided as it will take time and money to repair. It is easily possible to fit the sludge hose down the toilet pedestal, but if there is significant rubbish inside the latrine it is useful to be able to lift this out with a rake: something not possible through the pedestal. Exactly how this can be achieved will depend on the type of toilet being emptied.
Personal Protective Equipment Protective clothing is necessary, but clothing that gives adequate protection and is also reasonably comfy to wear and work in can require a compromise, particularly as most pit latrines are found in hot countries. It is also important that each emptier has their own protective equipment, as sharing masks could be sharing Tuberculosis, and the inside of gloves are only as clean as their owner keeps them.
Gloves Originally elbow-length gloves were used for emptying (Figure 49). These are tough and seem to last well, but during the testing of the eVac it was regularly necessary to take them off to do fiddly things such as adjusting things with spanners and using a camera. In the process sludge could be transferred to the inside of the gloves and they were then very difficult to clean. As a result we moved on to using disposable gloves. These gave more dexterity and could be thrown away when dirty, but are not tough and are liable to ripping. For this reason some sanitising hand cleaner is necessary.
Face Protection Initially a plastic moulded face mask with filters for organic material was used but these were uncomfortable to wear and so disposable face masks were tested. Although they have an ongoing replacement cost, they are considerably cheaper than the plastic masks and may be more hygienic. They are also lighter and generally more comfortable to wear. Goggles were initially used for eye protection but these were uncomfortable to wear for an extended period of time and the inside of the goggles steaming up was a problem. As a result a switch to safety glasses was made (Figure 49). The masks and glasses do not prevent splashes on the rest of the face, and so visors (Figure 50) are being tried as a potentially more suitable solution.
Overalls & Shoes Although overalls are an obvious choice (Figure 48), during hot weather they can be very uncomfortable to wear and are typically soon removed. In such circumstances it may be a good idea to offer butchers aprons or similar as an alternative. These also need to be regularly washed. Sturdy gumboots (wellingtons), possibly with steel toecaps, are also required. These will get dirty so a safe place to store them in transit is required.
Figure 48 â€“ Dirty overalls and disposable gloves
Figure 49 â€“ Elbow length gloves and safety glasses
Figure 50 â€“ A visor protects the face 61
Equipment required In addition to the eVac and the appropriate personal protective equipment there are several other things a pit emptier will need. Unless operating over a very small area, some sort of transport for the eVac and emptiers is required. Pickup trucks (Figure 51) are ideal vehicles for the purpose as they are very easy to clean. Where these are not available, the use of a trailer or small van may be appropriate. A power supply of some sort is necessary. This may be a local electrical connection or a generator taken with the eVac. If the latter the case it is desirable to leave the generator in the bakkie, and to take enough extension cord to reach the emptying site. Other items required include: Swimming pool chlorine pellets Plastic bags for covering/carrying contaminated equipment. A shovel for covering any spilled sludge Hazard tape for surrounding any exposed pits. Stakes and a hammer for putting up the barrier tape. Tools for removing the slab, where appropriate. Spare petrol for the generator Extra oil for the eVac Hand Sanitizer A rake, or similar, for removing rubbish from the latrine
Figure 51 â€“ A pickup truck is suitable for moving the equipment over long distances
Figure 52 â€“ A typical scene at the emptying site 63
Emptying Procedure There are different ways of organising a pit emptying operation, but in most cases it will be useful to visit all sites in advance to determine how full the pits are, whether they can be emptied and where the sludge will be disposed of. On this visit the lining of the pit should be checked, as unlined pits are liable to collapse when emptied. Using plastic covers on the ground to catch any spillage may seem like a good idea, but experience has shown that they attract dirt and are then difficult clean or move without spreading the mess around. It is better to accept the possibility of some spillage and then use a chlorine solution to disinfect the area, covering with soil afterwards. Covering with soil alone is not enough, as some of the parasites in pit latrine sludge survive well in soil. Donâ€™t use their tap! Washing anything using the householders tap will simply result in spreading the contaminants around. Care must be taken that any hoses that have had sludge in them are empty of all sludge before being moved, as we have often moved them only to find that some more sludge then dribbles out the hose. Cheap plastic bags tied around the end of the hoses make for a reasonable solution, however the sludge hose may have been into the pit latrine to some depth, so great care must be taken when handling. It is recommended that the sludge hose not be removed from the container lid until a suitable spot for cleaning is reached, as this tends to be a messy operation. If moving between sites then this is better left on for the whole day. The same applies to other hoses: the more bits that are touched the more chance there is of spreading contamination. A dedicated cleaning area for the eVac and its parts at base is desirable. For example, Partners in Development have a standard bathtub installed outside solely for the purpose of cleaning the eVac and its parts.
4. Costs Construction Cost The estimated cost to build an eVac is given in Table 1. These are in 2011 prices for parts sourced in South Africa and include VAT. The cost of a generator and a vehicle to transport it, should these be necessary, are not included. The cost includes three containers for the suck only eVac, and the one container for the suck and blow eVac. Twenty hours assembly time has been allowed for, at the rate of R50/hour. The trolley unit would be cheaper if greater numbers were being made, and discounts could be achieved on other parts as well. These costs are the based on the prices paid for parts for the prototype unit. Table 1 - Estimated cost to build an eVac Suck Only
Suck & Blow
Operating Cost Costs incurred during a pit latrine emptying operation may include: Management, administrative and office costs Advertising, in the case of private enterprises, or community liaison, in the case of public emptying Equipment purchasing Health interventions Labour costs Fuel costs incurred from transport Fuel costs incurred from emptying Machinery maintenance Repairing of latrines that had to be damaged to be emptied Disposal of removed sludge Tax It is not possible to determine a single cost per pit of using the eVac as it depends strongly upon the situation in which it is used. Prices for all of the above factors must be determined, as well as the crucial factor of how long it takes to empty a pit latrine: this will depend very much on the local situation. Table 2 demonstrates how variable the operating cost could be and that a thorough analysis is necessary for any given situation. Table 2 â€“ Example costs for emptying a pit latrine Cost per day Management, administrative and office costs Advertising, in the case of private enterprises, or community liaison, in the case of public emptying Health interventions Labour Fuel used for transport Fuel used for emptying Machinery maintenance Repairing of latrines that had to be damaged to be emptied Disposal of removed sludge Total cost per latrine 66
Optimistic (5 latrines/ day) R 500
Pessimistic (2 latrines/day) R 500
R 20 R 500 R 100 R 50 R 30
R 50 R 800 R 150 R 100 R 50
R R 280
R 150 R 1,100
5. Bibliography “What Happens When The Pit is Full?: Developments in On Site Faecal Sludge Management (FSM)” Seminar Report, March 2011, South Africa “Tackling the Challenge of Full Pits, Volume 3: The Development of Pit Emptying Technologies” (2012), Partners in Development and the Pollution Research Group, which is to be published as one of the outputs for the South African Water Research Commission project K5/1745. Search “eVac pit latrine emptying” on youtube.com for videos
Appendix I: Cost Breakdown Item 1.5 kW Electric Motor Pump Oil Lubrication Assembly Time Trolley Unit Trolley Steel & Fabrication Belt Drive 1" Air hose 1" Nipples 1" T 0.5" Pressure release valve 1" to 0.5" adapter 1" Camlock fittings Moisture Trap Clear PVC pipe for moisture trap End Caps for moisture trap Squash Ball for float valve 2" PVC pipe for float valve 1" Tank connector 1" Brass check valve Containers Roto-moulded container Suck only steel lid Suck & Blow steel lid Rubber Seal Outlet steel 3" nipple Outlet nipple attachment 3" Ball valves 3" steel elbow 3" Camlock 1" Camlock fittings Squash Ball for float valve 2" PVC pipe for float valve 1" nylon running nipple 1" to 2" adapter 1" T 1" Nipple 1.5" wide webbing Large Hose Clamps 1" 3 way ball valve Hoses 1" Air Hose 1" Jubilee Clamps 1" Camlock 3" Heliflex Hose 3" Layflat Hose 3" Jubilee Clamps 3" Camlock 3" PVC bushing
Quantity Suck Only 1 1 1 20
Amount Suck Only R 2,000 R 6,090 R 1,263 R 1,000
Suck & Blow R 2,000 R 6,090 R 1,263 R 1,000
1 1 0.5 1 1 1 1 2
R R R R R R R R
3,000 150 10 52
R R R R R R R R
3,000 150 10 6 30 750 20 52
1 2 1 0.25 3 1
1 2 1 0.25 3 1
R R R R R R
220 248 20 25 93 100
R R R R R R
220 248 20 25 93 100
R 500 R 1,000 R 1,000 R 20 R 50 R 400 R 750 R 100 R 50 R 26 R 20 R 100 R 25 R 10 R 30 R 10 R 10 R 40 R 700
3 1 0 1 0 0 0 1 1 1 1 1 1 1 1 6 6 0
1 0 1 0 1 1 2 1 2 2 1 1 1 1 1 2 1 1 2
R R R R R R R R R R R R R R R R R R R
1,500 1,000 20 100 50 26 20 100 25 10 30 60 240 -
R R R R R R R R R R R R R R R R R R R
500 1,000 50 400 1,500 100 100 52 20 100 25 10 30 20 10 40 1,400
R R R R R R R R
5 4 4 5 0 1 1 1
10 6 6 5 7 2 2 1
R 100 R 40 R 104 R 480 R R 27 R 50 R 50 R 18,303
R R R R R R R R R
200 60 156 480 154 54 100 50 21,688
Unit Sum Sum Sum Hours
Rate R 2,000 R 6,090 R 1,263 R 50
Sum Sum Metres Sum Sum Sum Sum Sum
R 3,000 R 150 R 20 R 6 R 30 R 750 R 20 R 26
1 1 0.5 0 0 0 0 2
Sum Sum Sum Metres Sum Sum
R R R R R R
Sum Sum Sum Metres Sum Sum Sum Sum Sum Sum Sum Sum Sum Sum Sum Sum Metres Sum Sum Metres sum sum Metres Metres Sum Sum Sum
220 124 20 100 31 100
20 10 26 96 22 27 50 50
Suck & Blow 1 1 1 20
Appendix II: eVac Field Test 2 21st December 2011 After the success of the initial test with the eVac, a drier pit latrine was found for a further test. The design of the latrine meant that the latrine appeared full from the toilet seat, but was actually nearly empty. The eVac struggled to lift the very dry waste and it was found that using the air/water lance increased the pumpability somewhat. Further tests on latrines that really are full are required.
Changes to the eVac since the last test Although no major changes have been made to the eVac, several minor changes have been: A pressure gauge has been fitted to the suction pipe coming from the eVac so that the vacuum achieved can be monitored. Two extra containers have been made to allow for a more efficient container emptying process. Two extra hose clips have been used to attach the hose onto the pole which controls it, bringing the total to four. The cable ties holding on the moisture trap have been replaced by hose clips. Two changes to the protective equipment used were: 1. Rather than using the tough elbow-length gloves when using the machine, we trialled using disposable plastic gloves, which were replaced regularly throughout the trial. 2. A large number of cheap plastic bags were used to place dirtied equipment until it could be cleaned back at the office.
The Pit The pit latrine consisted of a small, low superstructure with a large plastic toilet seat. The pit outside was covered by a corrugated iron / concrete slab. The superstructure was cramped and not high enough for taller people to stand up straight in.
The pit was offset from the superstructure, and a chute directed waste appropriately. This was not immediately apparent, however.
Sludge Figure 53 â€“ Simplified representation of sludge in pit
Figure 54 â€“ (left) The sludge pipe and air hose in the pit; (right) the consistency of the sludge removed from the pit
The Test Initially the suction pipe was put through the toilet seat. It was almost impossible to see what was happening as we had not brought a torch but the pipe seemed to get blocked twice. In hindsight this may have been us pushing the pipe into the clay soil, not realising that there was no pit below. We tried pouring in water to make the emptying easier. After pouring in around 250 litres of water there was no change in the consistency of the waste. The water seemed to be running into the main area of the pit rather than soaking the sludge we were trying to empty. Eventually we decided to open up the slab. It was not hard to open but a whole new slab has had to be re-cast. This revealed that the pit was almost empty, but that the waste had accumulated on the chute, making it appear full from the toilet seat. Simply poking it with a stick would have cleared this. Despite the emptying now being unnecessary, we decided to attempt to remove more. The eVac struggled to remove the more soil-like waste, even through plug and gulp. It seemed as though an air-tight seal with the sludge could not be formed to create the required vacuum, and that air would leak into the pipe. This was solved through the addition of water using the air/water lance. This meant that a relatively small amount of water could be added to the correct place, and the waste then became easier to lift. The current eVac configuration even allows for these two procedures to be done simultaneously. The addition of air-alone was not seen to have very much effect on the pumpability of the sludge. This may be because not enough air was being added, and so further investigation on subsequent pit latrines using a stand-alone compressor may be warranted. Despite the addition of water, progress was very slow. In total around five 45 litre buckets were filled and emptied. It is uncertain whether we were pumping up sludge the whole time or whether we may have been sucking up the clay from the bottom of the pit. (Ketha was ‘100% certain’ that it was the bottom of the pit we were sucking up, I wasn’t so sure).
Other Points Some air could enter through the thread between the 3â€? 90 degree bend and the lid at times, because of torsion from the pipe. Whilst emptying through the toilet seat, the length of pipe that acts as a handle needed to be cut down considerably, as the area inside the latrine superstructure was very low. It was 36 0C. The protective equipment we wore made this even hotter, and as a result it was not always worn properly. We used disposable gloves rather than the elbow-length gloves used previously. These were nice to work in but could tear. We forgot to take plastic sheeting for the ground with us. Despite this there was very little mess, helped by using two containers so that the lid could be moved directly between them. The positive pressure release did not function well. In the heat the soft rubber became too soft and it is now quite ineffective. The problems encountered meant that few containers were filled. This means that the presumed increased efficiency of using multiple containers was not realised.
Next Steps Test on more pit latrines. Preferably dry ones that are actually full! Potentially use an air compressor if the air lance proves not to be powerful enough. Continue developing a container which can suck and blow the waste, rather than suck only with manual emptying. Weld the connections onto the lid to reduce air loss there. Investigate how the positive pressure release valve could be improved.
Appendix III: eVac Field Tests 3+4 March 2012 Following mixed success for the eVac in previous trials it has been tested on a low flush toilet pit and another VIP latrine. In both cases the eVac managed to suck the watery waste out without difficulty, but because of splashing and spillages it made a significant mess around the emptying area. An emptying methodology that creates less mess must be used in future.
Changes to the eVac since the last test No major changes have been made to the eVac. The sludge hose connection on the lid has been welded on to prevent air leakage or unscrewing during use.
The Pits Two pits were emptied. The first was from a low flush latrine in France, Pietermaritzburg. This pit was situated at the back of the house, whilst the pedestal was inside the house. This meant that there was plenty of space in which to work. The homeowner had dug a disposal pit, but this was on the other side of his garden, as far away from his house as possible. Unfortunately the only way to move between the two pits was to walk across his vegetable patch. The second pit was very different; it was a pit latrine with an offset pit, covered with a plastic dome. Having previously found that offset pits could appear full, whilst actually having lots of space inside them, the sludge was initially prodded with a long stick and water poured on it, but it appeared that the pit was indeed full (Figure 55). Access to the pedestal was easy, as the latrine had a standard superstructure, but access to the pit from outside the superstructure was near impossible, as the plastic dome has a lip which is embedded in the foundation of the superstructure. The homeowner had a dug a large disposal pit about a meter away from the latrine, which they had used corrugated iron to cover.
Figure 55 â€“ (from top) The house where the first pit we emptied; their vegetable patch with disposal pit behind; a plastic dome shaped pit cover; the second latrine and covered disposal pit; the second latrine before emptying
The Test The eVac could suck the very wet waste from the first pit very easily. There were no problems whatsoever with the sucking process and the 40 litre containers filled in under 10 seconds. The critical factor in determining the speed of the emptying cycle was the time it took to walk with the containers to the disposal pit and back. Between two people the containers are quite easy to carry, but trying to avoid the cabbages in the vegetable patch made the process significantly slower and more difficult. Furthermore, the watery consistency of the sludge meant that it could splash onto the ground and onto the labourers when being carried and poured into the disposal pit. For emptying such a liquid pit the current methodology exposes the labourers and the homeowners land to too much sludge. A suck and blow tank is necessary for such liquid sludge, as this should reduce exposure. Alternatively a simple clamp on lid system could be used to prevent the sludge from spilling when the containers are carried. The second pit was a traditional VIP latrine and it was expected beforehand that sludge from it would be quite dry, as this had been the case at a nearby latrine. Once empting had begun, however, this proved not to be the case, and most containers filled in around 10-15 seconds, although some took longer. There was a moderate level of rubbish in the latrine but despite this the sludge pipe only blocked once, when a bag of something wedged itself in the end (Figure 56). This had to be removed using a spade. With only two labourers, the total cycle time for filling and emptying the container was close to a minute: with another labourer this could have been significantly faster. The pole that was attached to the end of the sludge pipe to control it had to be removed as it was not long enough to reach the bottom of the latrine, even with an arm stuck right down through the pedestal. The pole had previously been cut down to make it easier to handle in a latrine with poor access. The emptying of this latrine was not affected by its removal. Despite the watery nature of the sludge, splashing was not such an issue as it had been at the first pit. This was because the disposal pit was deeper and the container did not need to be carried. Despite this a significant mess was made of the area around the pits, caused by 76
spills from the container, sludge leaving the sludge pipe after the container lid had been removed, and the removal of the sludge pipe from the latrine. As it was not possible to remove the pit cover, it was difficult to determine exactly what was happening inside the pit (Figure 57). It appeared however that the pit was emptied in full. At neither pit was the air lance used, as the sludge was not dry enough to warrant it.
Next Steps Further work on the suck and blow tank. Use visors to protect the whole face from sludge splashing. Use swimming pool chlorine to make safe any sludge spills at future pits. Test on more pit latrines.
Figure 56 - The blocked sludge pipe
Figure 57 - The view from the pedestal
Appendix IV: eVac Field Tests 5+6 28th March 2012 With a new suck and blow tank another low flush toilet soak pit is successfully emptied, but the eVac subsequently fails to empty a standard VIP latrine, primarily because of the high rubbish content, coupled with poor access to the pit.
Changes to the eVac since the last test Alongside the creation of a suck and blow tank, one small change has been made to the removable lid: the 1” connection has been welded on to make it sturdier. This does however mean that the suction hose cannot now be rotated to face the most convenient direction.
Suck & Blow Tank Unlike the tanks used previously, the suck and blow tank is designed to withstand positive pressure as well as a vacuum. This meant that the lid could no longer be the easy-to-remove type used for the suck only tank, and is instead bolted on: the theory being that it should only rarely be removed (i.e. the outlet pipe should not block!). The tank itself is the same as the other LLDPE rotomoulded containers (Figure 58). This steel lid has a 3”elbow welded on for the sludge inlet, as per the removable lid, which can be controlled using a 3” plastic ball valve. Additionally there is a 1” hole in the lid for the air in/out-take, with an identical float-valve to that used with the removable lid connected below this hole. Connected to this 1” hole are a pair of 3-way ball valves, one for the suction line and one for the pressure line. Each of these valves can be set so that the line from the eVac connects to the tank or to the atmosphere. An outlet at the bottom of the tank connects to a 3” lay-flat pipe. This outlet also has a 3”plastic ball valve attached. The functions of both of the 3” plastic ball valves could be better served by duck bill valves, as these would not need to be turned by hand and would simplify operation. These could not be sourced in South Africa and although importing them is possible, the 3” valves currently used serve the same purpose and seem most appropriate for this prototype.
A pressure release valve has been purchased as success in making one has been limited, and it was felt that it was imperative to have effective pressure release given that a pressure vessel is now being used. This has as yet proved to be unnecessary, as air leaking from some of the bolts means that the eVac cannot currently pressurise the tank to greater than 1.5 bar.
Figure 58 - The suck and blow tank
Test Five Two pits were emptied. The first was a soak pit from a low flush toilet in France, Pietermaritzburg. This pit neighboured the house used for test three. A shallow disposal pit had been dug nearby by the family (Figure 59). As hoped, the eVac was able to suck up the sludge from the first pit very quickly. The filling time in each cycle was reduced even from test three. This can be attributed to the volume of sludge at the bottom of the tank which was not emptied out at the end of the cycle and the sludge inlet pipe remaining largely full between cycles. Blowing the waste into the disposal pit was also a quick process, as was switching the valves between suck and blow. The suck and blow tank requires 4 valves to be adjusted to switch from sucking to blowing and vice versa. Although this is not a difficult task and no damage will occur if operated incorrectly it is an area where confusion could occur and potential operational problems could arise. The current setup of the tank means that the 3-way air valves tended to rotate as they were used. A more robust setup, probably by welding the system to the lid, would be a better long-term solution. At this stage in development the flexibility in being able to change the setup is useful, however. The emptying was stopped when the disposal pit became full, the total time including setting up, emptying and packing away was around 45 minutes. Three labourers were used: one to control the sludge intake, one to operate the valves on the tank, and one to control the sludge outlet. One of the key areas to improve upon identified in the last update was to reduce the mess on site. The suck and blow tank was designed to improve this, but there were still problems. The sludge could leave the layflat pipe into the disposal pit ferociously, meaning that it was necessary for somebody to hold the end of the pipe. This did not prevent splashing, however, and lead to the person operating the sludge outlet getting a substantial amount of sludge onto their overalls (Figure 60). Furthermore, once the pit was emptied the pipes were removed from the suck and blow tank and a substantial amount of sludge drained from them onto the ground. A possible solution in future would be to keep the pipes on until a â€˜safeâ€™ area for cleaning is reached (in our case in the special outdoor bath at the office). 81
After the emptying had been completed the site of test three was inspected. The disposal pit which had been filled during the test had drained, leaving only a layer of fine sludge no more than 2cm thick (Figure 61). It was now being used as a rubbish disposal pit. Additionally, the soak pit was full with water again. This suggests that the soak pits are expected to be full, and that emptying these kind of pits is unnecessary. On the return journey the sludge exit pipe snapped off from the rest of the container. This should probably be welded on, rather than using a PVC pipe.
Figure 59 - The set up for test five
Figure 60 - Splashes of sludge are visible on Vincent's overalls
Figure 61 - The disposal pit from test 3, the solids content was very low
Test Six The second pit tested was in an area that has been established as having many full VIP latrines, predominantly latrines with a â€œwalk aroundâ€? entrance and offset pits covered with a concrete dome. This dome typically has a ~200mm access hole, and many have missing vent pipes, creating another smaller access hole (Figure 62). A survey found that there are at least sixty full pits in close proximity to one another, and the purpose of attempting to empty this pit was to determine if this would be a good area for a larger-scale trial. The pit tested was chosen because there was an existing large pit next to the latrine which could be used for disposal of the sludge. The attempt at emptying the pit was made with the removable lid suck-only system (Figure 63). Accessing the pit through the dome at the back of the latrine, the eVac failed to suck any substantial amount of material from the pit, despite being able to build up high vacuum pressures using the plug & gulp method (Figure 63). This seemed to be due to the substantial rubbish content of the pit. If the dome shaped pit cover had been removed it may have been possible remove the rubbish with a rake, or similar, and the pit could then have been emptied. The cover was not attached to the superstructure so removal may have been possible (with difficulty) however the holes in the dome had been used for putting rubbish down, and possibly as a toilet themselves. This meant that the sludge had built up inside the dome and so removing it would have caused a huge mess. A neighbouring pit latrine, also within range of the disposal pit, was considered but the pit looked as much like a landfill site as it did a toilet. One further latrine was visited, where the owner had dug a new pit with the intention of building a new latrine. The new pit had filled with water and she had now applied for a new latrine instead. She did not want us trying to empty her latrine as it may jeapordise her chance of getting a new one.
Figure 62 â€“ The concrete dome, with access hole and a hole where the vent pipe used to be
Figure 63 - Test six
Figure 64 - The total sludge emptied during test six
Published on May 10, 2012
The eVac is a portable vacuum machine for emptying pit latrines. Safer and easier than emptying pit latrines by hand, it is cheaper and more...