Costs: Treatment of domestic wastewater

Collecting and treating domestic wastewater can be quite a problem in rural areas or small communities that are remote from municipal treatment plants. One traditional alternative to mains sewers is to collect wastewater in cesspools, or storage tanks, then tanker it away for remote treatment. Another is to use a septic tank, in which the wastewater is settled out and then partially digested by natural biological processes before being discharged as a liquid effluent into a soak away specially created for that purpose. However, in recent years, package plant and filtration systems have become more established as alternatives.

Package plant systems

Package plant systems generally work by introducing air into the collected wastewater, either by means of a pump or by dipping slowly rotating discs into the water. They are effective at providing primary and secondary wastewater treatment, although they do consume electrical energy and can be unsightly. On the other hand, due to the higher quality of treated effluent produced, the infiltration system used with a packaged plant can be smaller than that for a septic tank. A packaged plant with an infiltration system will normally occupy the smallest area, so might be an appropriate specification if space is limited.

Reed bed systems

A more environmentally friendly alternative to a packaged plant is a constructed wetland system. Such systems try to mimic natural reed beds, which have shown to be effective in treating contaminated water. Reed bed filtration systems and other wetland-based processes have become popular in recent years. They benefit from low cost and energy requirements, simple operation, and a number of environmental plus points. Large systems encourage biodiversity - extensive reed beds are among the most important habitats for a variety of rare birds, invertebrates, flora, fish, amphibians, reptiles, and mammals. From a human perspective, they can encourage local and traditional trades, such as basket making.

Two types of constructed reed bed are used in the UK, the horizontal bed and the vertical bed. Although their methods of treating wastewater are different, they can produce similar qualities of treated water. Vertical beds are much smaller in area, and are best to specify on a sloping site. Horizontal beds can require considerable areas of land, but are simpler to construct. Usually the top surface is planted with reed, but, as one of the main treatment methods in a reed bed is filtration, a variation is to use unplanted beds that are simply sand filters. The depth of most reed bed systems is about a metre as the reed's roots and rhizomes is rarely beyond 0.6 m. Most contaminants are removed by filtration, and other processes, within the first 0.5 to 1 m of soil.

All other dimensions of a bed are dependant upon the amount of wastewater passed through it. Horizontal beds work mainly by anaerobic treatment that works without air but takes long enough for weeds to grow. Vertical beds mainly work by aerobic treatment and the discharge pipe dimensions, along with the bed area influence the overall size, but more frequent discharges can be applied to accommodate greater loads.

Specification options

Five options are considered below for treating domestic wastewater. The prices in the cost model are for the design and installation of a wastewater treatment system for a rural eight-person house. For other-sized dwellings these same systems can be considered.

There is a difference in component and installation costs for the various options. The running and maintenance requirements for the systems are also different.

Assuming no pumping is required for any system, as all flows are by gravity, the only system requiring constant electricity is package plant. As for servicing, septic tanks need regular (typically biannual) desludging by tanker because of their mechanical and electrical plant. Package plants may need more frequent servicing.

Standards, guidance and regulations

• Basic design of wastewater treatment systems is currently specified in BS 6297:1983 (to be replaced by EN 12566 when all of its parts have been published).
• EN 12566 includes specifications for Septic Tanks, Package Plants, Infiltration systems and Filtration systems.
• Design guidance for Reed Beds can be found in the BRE Good Building Guide GBG 42, parts 1 and 2 and the BRE report BR 420 Reed Beds for the Treatment of Domestic Wastewater by Grant and Griggs.
• Minimum requirements for small wastewater treatment systems are set out in the Building Regulations: in England and Wales, Approved Document Part H2; in Scotland, Sections 3.8 and 3.9 of the technical handbooks; and in Northern Ireland, Regulations Part N6.