Q
odour management
refereed paper
This n otwithstanding, a brief review of devices for mitigating the odou r impact
Table 2. Preventative Sulfide Control Technologies.
of foul air released from AVs was requested and undertaken. Reviewed devices included filters mounted at the end of vent stacks, ground-mounted fi lters, an in-g round biofilter, a dry scrubber for manholes, as well as the convent ional drum dry scrubbers. They were all assessed in regard to: capital and operating cost; required services and vehicle access; ease of retrofitting into the existing pit ventilation arrangement; ease of replacement when exhausted; service life; robustness; and resistance to passing the highest antic ipated flowrate of ai r.
Technologies
Advantages
Disadvantages
Oxygen dosing
Relatively simple application Proven technology
Promotes the removal of readily degradable BOD Complex delivery system for effective oxygen transfer Injection does not lead to effective oxygen transfer Cannot be easily relocated Medium/high capital costs Medium operating costs Reaction time is pH dependent and may be slow Need to consider dosing location carefully 02 can accumulate at high points and cause corrosion. Also, risk of explosion is increased Works best under high pressure rising mains Doesn't work well unless flow is semi-continuous Liquid oxygen is hazardous
Magnesium Hydroxide (MHL)
Non-hazardous Increases the alkalinity and reduces the SAR of wastewater Non-corrosive Low/medi um capital cost
Cost is directly proportional to flow treated Single supplier Limited shelf life (3 months) Effectiveness declines as salinity increases Tends to settle to the bottom of PS wet-wells and form a gelatinous mass there Frequent flushing of dosing lines and mixing of storage tank contents is needed to prevent blockages Effect on struvite production at treatment plant is not known
Ferric Chloride (FeCl 3)
Low/medium capital cost Longer shelf life than MHL Handles high sulfide loads Rapid reaction
Hazardous Corrosive Medium operational costs Does not oxidise non-sulfide odorous compounds Consumes alkalinity so alkali addition is often required Dosage increases with sulfide concentration Wi II also react with phosphate Creates a sediment and may have impacts on sludge dewatering and handling at the STP Contains metals other than Fe
Calcium nitrate
Safe handling Suitable for most systems, particularly when dosed prior to sulfide formation
Promotes the removal of readily degradable BOD Increases total-N load, which can be an issue for downstream wastewater treatment plant Stimulates consumption of organic compounds that assist denitrification at the WWTP Potentially high operating costs Complex dosing control In some cases may promote precipitation of CaC03
It should be noted t hat the combination AVs not only release air but may also admit air, and that t he air flowrate may sometimes be large, e.g. during p ipeline fil ling. It was fou nd t hat the suppliers o f most of th e reviewed odour treatment devices were either unable or unwi lling to describe the relationsh ip b etween pressure d rop across the device and the air flowrate passed by the device. Consequently, it is not yet clear how many of these devices have to be arranged in parallel so as to allow t he AVs to function as intended. With some qual ificatio ns concern ing third party safety, OH&S, back pressure, and filter warm ing due to reaction with substances in the foul air, it was recommended that the brand of vent stac k fi lter preferred by Unitywater be retained, where that had been employed.
Assessment of AV locations Once it had been established that hydraulic entrainment could not eliminate enduri ng gas pockets from the CRM, the meth odology in Section 10.9.3 of Part 1: Planning and Design of the Sewage Pumpin g St ation Code of Australia (WSA 04-2005, WSAA) was em p loyed to determine appropriate locations for AVs to reduce corrosion risk . This section of t he Code states that: "Gas release valves shall be fitted at all high points on the pressure main unless it can be demonstrated that the flow velocity is sufficient to move accumulated gas at the high point downstream to the next high point in the pressure main or point of discharge." WSAA a lso specifies that automatic combi nation AVs be used as gas release valves for sewerage rising mains. It was fou nd t hat all the existing com bination AVs were appropriately located to vent air from the CRM, but
64 DECEMBER 2010 water
that combination AVs were requi red at an additional 10 locations. At four other locations AVs would be required for corrosion protection, but for the prediction of hydraulic entrai nment. A conservative approach was taken of recommending that manual AVs be installed in pits at t hese four locations: the AVs to enab le the absence of air pockets to be confirmed, and the pits to facilitate the monitoring of wall t hickness.
Chemical Dosing Options Having reported on means for reducing the risk of corrosion through operat ion of appropriately installed AVs, we developed o ptions for f urther reducing the risk o f corrosion with in the CRM by means o f chemical dosing. Th is options development was preceded by sewage
characterisation at selected points along the CRM, and by application of steadystate models for the daily average total sulfide concentrations in the current CRM and in the upgraded CRM. For t he sewage characterisation, sewage monitoring was undertaken during a 24 hour period in November 2009 at p ump stations and selected scour valves. The characteristics thereby obtained were typical ones for d omestic sewage, with the exception o f the values reported for the single pump station catchment receiving significant amounts of trad e waste. The sulfide model is based on the methods, equations and advice provided in the Hydrogen Sulphide Control Manual (1989) (HSCM), on recent literature, and GHD experience. Model inputs