Drinking Water Treatment
The slow sand filtration alternative Water treatment for small systems flow,it must then be partially drained and the schmutzdecke scrapped off and discarded or stored for cleaning and recy cling. Filter 'runs' of one to several months are typical. Over time the sand bed depth is reduced from the scrapings and new or recycled sand must be refilled into the box. The major advantages of slow sand filters are: • Effective treatment for turbidity, bacteria, and greater than 4 log removal of cysts. • No pre-treatment chemicals. • No backwashing. • No automation required. • A failure to clean the filter will result in loss of production but no loss of quality. The major disadvantages of conventional slow sand fil ters are:
Filter under construction.
In the late 1980s and the early 1990s there was a grow
ing emphasis on appropriate technology for small water treatment systems. Entire workshops and semi nars were dedicated to advancing the knowledge that small systems had to be simple and affordable to operate. This lesson was learned the hard way in countless sad cases of complicated mechanical and chemical systems which no local staff could maintain and operate efficiently. Unfortunately, it appears today the principle of appro priate technology has become a casualty of the headlong rush to high technology. However, more and more authori ties are asking who is going to pay for these plants with high annual operating costs and when their obsolescence sets in, a period now measured in a few years, rather than a few decades. This inevitably leads to a search for a simpler, more cost-effective, and more durable solution to the small
communities'servicing needs. Maybe the search should start with our lessons of the past and improve on the systems to make them more adaptable to today's and tomorrow's water quality needs but equally durable and simple to operate. Conventional slow sand filtration
The traditional slow sand filter is basically a box full of sand. Figure 1.1 shows a cross section of a 'conventional' filter. Water enters the filter compartment above the media and flows down through the sand and, in time, will form a thin biological layer called a schmutzdecke. The combination of physical strain ing and biological treatment will effec tively remove turbidity, bacteria and will remove in excess of four log of Giardia cysts and Ciyptosporidium oocysts. As the bed plugs up,the water level will rise over the sand. As it approaches an es tablished upper level, usually the over-
• Raw water turbidity must generally be low. • Raw water must not have high algae counts. • Colour removal is fair to poor. • Unpredictable filter run times necessitate lengthy pilot studies.
• Need for a conservative design in the event of high algae results in very large filters. Several conventional slow sand filter plants exist in Ontario and British Columbia. A small plant in Serpent River, Ontario, has been operating since 1987 with results as indicated in Figures 2.1 and 2.2. The filters provide excellent turbidity removal and pro vide the community with a safe and low maintenance treat ment system. A pilot study was required and determined that the filters needed to be larger than initially expected. Finally the plant does not always remove colour to below the Province of Ontario aesthetic guidelines despite the fact that the Serpent River raw water quality is relatively good compared to most surface water sources. Improving the slow sand process In summary, the shortcomings of the conventional slow sand filtration process are: • Short filter runs or need for very large filters if raw water has high turbidity or algae. • Fair to poor colour removal. • Lengthy pilot study is often required to predict fdter run time.
A number of enhancements have been studied to improve
water
Overflow Schmutzdecke
Vent
Sand Filter Effluent
By Robert A. LeCraw, P. Eng., RAL Engineering Ltd. 42
FIGURE I.I CONVENTIONAL SLOW SAND EILTER
Environmental Science & Engineering, May 2000