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<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>

b. Overland flow

Overland flow systems aerobically treat wastewater, providing secondary treatment and nitrogen removal. Pretreatment includes at least screening. Recent practice adds a short detention time aerobic lagoon upstream for solids removal and addition of dissolved oxygen. Public access to the application site is not allowed. The technology is mainly used in the southeastern and southwestern U.S. In cold weather climates, winter storage is required as treatment efficiency, especially nitrogen removal, is adversely affected by cold temperatures.

c. Wetland systems

Wetland systems are used to treat primary and secondary effluent, especially when discharging to a sensitive environment locations. Typical wetland plants include cattails, reeds, rushes, bulrushes and sedges. The plants are not usually harvested, though in some systems they may be periodically burned off to maintain system hydraulics. Operation is possible in cold climates, although storage may be required and the effect of freezing must be allowed for. Constructed free water surface (FWS) wetlands can have a secondary purpose as wildlife habitat, although mosquito control is an issue. FWS wetlands are more common in the U.S. than subsurface flow wetlands. Subsurface flow wetlands have been used for residential sewage treatment in Canada since the early 1990s, but only since about 1995 in the US.

Natural wetland systems can and have been adapted for wastewater treatment but are more likely to be a final disposal point, since adaptation of the natural wetland may harm the local ecosystem.

d. Tank systems

Communities may have a single septic tank system followed by a large leachfield (i.e.: Taylorsville, California). Large septic tanks are used upstream of mechanical treatment, such as rotating biological contactors in community sized installations. Imhoff tanks have been used for small communities in the past (U.S. EPA survey in 1988, flows of about 0.95 MLD, 415 facilities) however they are now prohibited in some states. Operational problems include odour problems and production of odorous foam and sludge.

e. Mechanical treatment plants

All of the technologies in use for single family dwellings are available in larger scale for use in small communities. The usual refinements include positive sludge and scum removal from clarifiers, and sludge holding tanks which, if aerated, also act as mesophilic aerobic digesters. Installation of rotating fine screens upstream of the mechanical treatment plant decreases buildup of floatable material.

Multiple tank systems with recycle, such as the Biogreen system imported from Japan, are claimed to be capable of up to 50 percent nitrogen removal.

In this size range, recirculating intermittent sand filters are used rather than the single pass systems used for individual homes.

Sequencing batch reactors (SBR) are economically competitive over 150 m3/d, and have been used with success in many locations throughout the US and Canada. SBR's have computer control allowing easy manipulation of react, settle and decant times to allow for changing inlet conditions.

Remote monitoring of community-scale plants by telephone and by satellite is available and becoming more common, allowing a central monitoring station by a manufacturer to contact a local operator and assist with maintenance. Remote monitoring of single family dwelling systems is possible, but is not generally done due to the high cost of equipment and telephone lines.

Besides secondary gravity settling, other solids separation technologies used at this scale include dissolved air floatation, and membrane filtration.

Advances in dissolved air floatation include improved air entrainment using venturi devices which enable equipment manufacturers to use shallower tanks than conventionally possible. These systems, such as the Hydroxyl Systems Inc. Positive Floatation Mechanism (PFM), are also finding application as primary and secondary solids “clarifiersE and research suggests that the highly aerobic sludge cap at the surface results in accelerated solids destruction and denser sludge in such applications (Figure 4.3).

Figure 4.3: Hydroxyl Systems Inc. Positive Floatation Mechanism (PFM)

Membrane filtration technology operates either downstream of high solids concentration activated sludge secondary treatment, or within the aerated bioreactor of such systems. Effluent may be either forced by pressure through wound tubular membranes, or drawn by light suction through the membranes using vacuum pumps. Nominal pore sizes are typically small enough to filter out bacteria, but cracks or breaks in the membrane may permit bacteria to cross over the membrane barrier. An example of a commercially available membrane used in a wastewater treatment membrane-bioreactor process is the Zenon Environmental Inc. ZeeWeedŽ membrane system illustrated in Figure 4.4 []. Computer controlled pressure variations and/or periodic backwashing help to keep the membranes clean. Nitrification is generally good due to the very long sludge ages achieved in membrane systems, but denitrification and overall nitrogen removal depends on the system design and presence of anoxic/anaerobic reactors within the process. Phosphorus removal I membrane systems typically requires chemical addition.

Figure 4.4: Zenon Environmental Inc. Membrane Bioreactor System
And ZeeweedŽ Membrane

Physical/chemical treatment using chemical flocculation was practiced in the past to treat wastewater with widely varying flows and loads, especially high loadings. Greater understanding of biological wastewater treatment, complex operational and maintenance requirements, and more stringent requirements for chemical sludge disposal has resulted in fewer installations of this type. However, there are still instances where biological wastewater treatment may not be practical - for example, where a mobile unit requires instant treatment without a startup time allowance. Electro-flocculation has been applied to industrial processes in the past, and is now being developed by companies in Canada, and the US, for use with wastewater treatment.

Figure 4.5: Solar Aquatics Treatment System (lager image)

One technology (developed in Massachusetts by Solar Aquatics, and illustrated in Figure 4.5) combines activated sludge and constructed wetlands in a package plant located inside a greenhouse.

f. Odour control

Especially for small community installations located near homes, odour control can be an issue. Technologies used include activated carbon filtration, chemical scrubbing and biofiltration. Biofilters for odour control are above ground piles of composting material (often woodchips) with spray wetting systems and underdrains to collect filtrate. Modular biofilter systems consisting of stackable trays have also been developed.

g. Grease traps

Simple grease trap tanks have been generally used on wastewater discharges from restaurants, laundromats and service stations to prevent plugging of the downstream treatment system. A change to use of low temperature soluble vegetable oils rather than animal fat has resulted in these traps being less effective. Solutions have included the NIBBLER, a combination of attached and suspended growth secondary treatment technology developed in Oregon, and enzyme additives [Nothwest Cascade-Struth, Puyallup, Washington WA 98373; ph: 800 4442371]. Although the technical basis for such additives is questionable, some users of additives have reported good results.

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