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4.3 Treatment (Topic c)

4.3.1 Small scale and community scale technologies

In the following discussion, small and community scale is defined as from 1 to 5000 Population Equivalents.

Stormwater

Swales, wide shallow grassy ditches, are used to direct stormwater flow around a surface structure, and provide some flow attenuation, filtering, and evapotranspiration due to the grass growth, as well as some minimal storage. Ponds are used to contain stormwater flow, hold flows until pollutants are biologically broken down or adsorbed onto particulate matter, and often allow water to infiltrate into groundwater gradually through the pond bottom. Sedimentation and filtration ponds may also be lined to facilitate removal of accumulated sediments, usually every two to four years. Ponds and swales are also effective in removing sediments from storm discharges, but are land intensive.

Ponds are also used to treat runoff from parking lots. These systems may also include upstream filtration provided by grass plots underlain by sand and gravel filters with an underdrain system feeding the holding ponds.

Natural, enhanced and artificial (constructed) wetlands are also used to treat stormwater runoff before discharge to the environment. Application is similar to that discussed under wastewater treatment below. Water may be diverted through a natural wetland system to take advantage of adsorption phenomena in the removal of sediments, and inorganic contaminants (metals). These natural systems may be enhanced to optimize the level of treatment and water attenuation.

Stormwater treatment can also be achieved in systems consisting of rock berms and alternating strips of native grasses and stiff grass hedges constructed on slopes. The rock berms are intended to catch coarse sediment and floatables, and convert piped flow to sheet flow down the slope. Native grasses provide screening, vegetative filtering and infiltration. Coarse hedge forming grasses provide flow attenuation to prevent flattening of filter grasses. Mowing and trimming of the site removes nutrients accumulated in the vegetation. This type of system has been applied in Austin Texas as part of a new subdivision, and requires less space than ponds.

Where space is at a premium, or where the groundwater table is high, radial flow filter cartridges installed in manhole structures have been used. One manufactured type, the StormfilterTM, has been installed in more than 130 sites in the U.S., including shopping centers, restaurants, highway runoff, hospitals, and single retail outlets [http://www.stormwatermgt.com/products.html]. Runoff from impervious surfaces such as roads and parking lots are treated by humic filter media composed of pelleted composted deciduous leaves. Up to 90% removal of solids, 85% removal of oils and greases and over 80% removal of heavy metals are claimed for such systems.

Small versions of vortex grit and floatables removal systems, such as the Downstream Defender (Storm King) or Vortex Separator (US EPA Design Criteria), have been used as a pretreatment stage for discharges prior to discharge to a gravel leachfield [http://www.hil-tech.com/dwnstrm.html]. Generally, there has been little detailed testing of the benefits derived from these large scale vortex flow devices. Research data indicates these vortex style systems typically remove only the coarsest fraction of the suspended solids, whereas toxic contaminants are more typically associated with the finer particulate fraction.

Catchbasin inserts have also been used to provide runoff treatment from debris control using mesh bags and screens, to oil and heavy metals removal by filter absorption. Rigorous catchbasin cleaning schedules become very important to maintain the drainage capability of the system. Simple oil traps such as submerged outlets on catchbasins are also used. The use of polypropylene sponges are recently being commercially promoted for use in catchbasins to absorb oil contained in the runoff. The hydrophobic oil containing sponges are periodically collected and processed.

Best Management Practices (BMPs) are encouraged on industrial and construction sites to prevent stormwater contamination at site. They include covering piled materials, roofing equipment storage and maintenance areas, covering drum storage systems, routine cleaning of paved surfaces, use of sedimentation basins and straw filters to trap runoff associated sediments migrating from the construction site, and regular inspections of potential problem areas. Drain blockers can be used in maintenance and equipment wash areas. On construction sites, erosion and subsequent large solids loads to the stormwater collection system can be controlled using methods such as Ditch DamsTM, a proprietary product composed of wire mesh and geotextile moulds which are filled with dirt and ready-mix cement on site [http://www.gullywasher.com/ditchdam.html].

Wastewater

Individual toilet systems

Pit toilets are generally used on remote, low density private and public recreational properties. Chemical toilets, whose contents are periodically pumped out by vacuum truck and transported to a central treatment facility, are widely used under temporary circumstances such as construction sites. Originally mainly used for remote recreational property, composting toilets (eg. Dowmus Composting Toilet illustrated in Figure 4.1) are now often used where a conventional septic system has resulted in water pollution, as well as for parks, beaches, highways, military installations, and golf courses [http://www.dowmus.com/home.html]. Imported from Sweden, North American modifications have included computer control functions, including positive mixing of the contents of the tank (Toilet Tronic, BC) and foam flush with an insulated composting tank for arctic regions (AlasCan Composting Toilet) [http://www.alascanofmn/index.html]. Incinerating toilet technology is used most often in mobile offices, boats, cottages cranes.

Figure 4.1: Diagram of the Dowmus Composting Toilet

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