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About UNEP
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United Nations Environment Programme
Division of Technology, Industry and Economics
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Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>

Individual homes

Wastewater technology for individual homes ranges from primary treatment in a septic tank, separation followed by ground disposal, to small package mechanical wastewater treatment plants. In some environmentally sensitive areas, especially those depending on tourist revenues such as the Florida Keys, nutrient removal is an important issue.

There is a performance standard for individual household wastewater treatment systems, referred to as the ANSI/NSF Standard 40. This standard was developed by the National Sanitation Foundation (NSF International) in Ann Arbor Michigan (a not-for-profit organization) on behalf of the American National Standards Institute (ANSI) to verify manufacturer's claims regarding performance. It is a voluntary certification testing program, which many Health agencies throughout Canada and the US have come to rely on. Testing under this standard is carried out in both Canada and the US at three locations: 1) Chelsea, Michigan; 2) North Saanich, British Columbia; and 3) Baton Rouge, Louisiana. Testing is carried out over a 26 week period, and involves monitoring treatment performance under standard loading and stress testing conditions.

a. Septic systems

Approximately one-quarter of all homes in North America are served by septic tank systems. Most tanks are constructed of concrete or fiberglass. Two compartments tanks are favoured due to more positive prevention of solids carryover to a drainfield. Tanks used as pretreatment before small diameter piping collection are fitted with filter vaults.

b. Biofiltration

There are many different kinds of biofilters which mimic the action of wastewater degradation in unsaturated soil, and are generally used where local soil conditions or development density preclude disposal in conventional septic tile fields. In addition to low BOD5 and TSS in biofilter effluents, fecal coliform counts are often significantly reduced, although this is not typically relied upon for final disposal due to the possibility of filter short-circuiting. The filters described below are usually used for single family dwellings but have also been applied for small home clusters. Effluent is generally nitrified, with approximately 50 percent removal of total nitrogen, but with the exception of peat filters, biofilters are not generally effective for phosphorus removal.

Intermittent sand filters are tending to be designed at lower loading rates and using finer media, to improve fecal coliform removal. Required piping, filter cloth, pumping equipment and controls can be purchased in a kit form for construction and installation by knowledgeable installers.

The Waterloo BiofilterTM (Figure 4.2), developed at the University of Waterloo, uses an absorbent plastic foam media and positive fan ventilation to allow loading at ten times the rate of an intermittent sand filter [http://oceta.on.ca/profiles/wbsi/bi.filter.html]. Nitrogen removal can be effected using recirculation, or by passing the effluent through a free draining sawdust box. Phosphorus removal is effected by passing the effluent through a ferric iron mineral filter. The Waterloo Biofilter has been installed in several locations in Ontario, one in Massachusetts, and one in Nova Scotia.

Upflow filter systems intermittently dose septic tank effluent below a bed of media, allowing solids to settle and decompose below the bed, while liquids rise through the media and treated effluent overflows over the top. The Glendon Biofilter, developed in Washington State operates in this fashion and the distributors claim it achieves a 10/10 (BOD/TSS) effluent. This system is typically designed for single home applications.

Peat filters treat septic tank effluent by gravity distribution by conventional leachfield piping over a buried peat filter. Sphagnum type peat is used. At least thirteen peat filter systems are in operation in Ontario, ranging from single family systems to schools. The filters have proven effective in reducing phosphorus, and BOD5 at appropriate loading levels. Nitrogen reduction is on the order of 40 to 60 percent.


Figure 4.2: Waterloo Biofilter System

c. Mechanical package plants

Single family dwellings have historically been supplied with either simple activated sludge plants or rotating biological contactors in integrated single tank systems. Newer technology combines both suspended and attached growth systems (moving bed biofilm reactors) with either fixed or floating media submerged in a suspended biomass. The combined systems are theoretically more resilient to shock flows and loads. These systems effect some nutrient removal, mostly in terms of nitrogen removal.

Solids separation is usually by clarifier, generally oversized by large system design standards to allow for wide flow variations, with no positive scum or sludge removal. Solids buildup is pumped out by vacuum truck at regular intervals and hauled to central treatment facilities. Biofilters have also been used with some success to improve effluent solids quality. Other types of solids separation systems, (such as ultra-filtration membranes) are available, but are generally not cost effective in single home situations; they are discussed in the following section.

Where single home systems discharge to a surface water body, disinfection is used. Options include chlorination/dechlorination using solid puck systems, single lamp ultraviolet disinfection systems, and ozonation. Ozonation is available as part of a package treatment plant developed by Hydroxyl Systems Inc, based in British Columbia, Canada [http://www.hydroxyl.com].

Small communities

Small communities use a variety of technologies from simple pond systems to more complex mechanical treatment plants.

a. Pond systems

Pond systems are popular in rural areas with available land area but low operational expertise. In 1984, there were 5298 stabilization ponds in use in the United States, with a further 2783 planned (Water Environment Federation; see 4.10 Sources of information).

In Canada, almost 9% of the population surveyed in 1996 with central sewage treatment was served by stabilization ponds. The actual number may be higher as communities with less than 1,000 persons were not surveyed ([http://www.ec.gc.ca/water/en/manage/vse/e-datab.htm] Environment Canada). Shallow aerobic ponds are used mainly in the southern part of the United States. Anaerobic ponds are generally used for strong industrial wastes rather than for municipal wastewater. Facultative ponds are the most common, and are in use in all climatic regions. Algae growth in these types of ponds can contribute to seasonal high solids concentrations in the effluent. Where wastewater temperature are sufficiently high enough during the summer, near complete nitrogen removal can be achieved (i.e. up to 95%).

In 1984, there were 1368 aerated lagoons in use, with a further 1494 planned (WEF). Aerated lagoons are often designed as several smaller ponds in series. A few more recent systems have provided high mixing in the first cell, followed by lower levels of mixing in subsequent cells to help prevent algae production.

Integrated lagoon systems combine facultative primary cells with fermentation pits acting as upflow anaerobic digesters followed by a shallow aerobic lagoon, with recycle back to the facultative lagoon. A system in St. Helena, California, has been operating for 27 years.

Lagoon discharges may be controlled to protect the receiving environment, discharging only when receiving stream conditions are satisfactory. Some lagoons, located where evaporation exceeds precipitation, can be designed as zero discharge.

Lagoon designs are tending towards more positive pond lining, including geosynthetic clays and geomembranes. Odour from anaerobic ponds is controlled using floating covers with gas collection.

Plant harvesting, a form of smaller polishing pond treatment practiced in warmer climates (air temperatures above 7 deg. C) involves growth and harvesting of floating aquatic plants such as water hyacinths and duckweed. Water hyacinth systems are in place in Headlands, Alabama, and San Benito, Texas. They have been used for nitrogen removal in Florida. Phosphorus removal is not practiced.

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