2.5 Rural Area Sewage
2.5.1 How is sewage from rural areas treated?
In rural areas, domestic sewage is normally dealt with on
individual premises in cesspools and septic tanks.
Cess-pools are tanks without an outlet that are used solely to
store the sewage. The stored waste must be frequently removed as the tank is
filled. Cess-pools are expensive to operate due to the frequent necessity to
empty them. Cess-pools are installed only when the ground is impermeable or the
water table rises to near or above ground level for all or part of the year.
Fig. 2.3 Factory-built treatment
plant for small, rural developments
Septic tanks are small underground tanks interred in the ground,
away from the houses that they serve and which act as small treatment plants
with a low efficiency. Solids settle to the floor of the tank and oils and
grease rise to the surface of the tank contents. A clarified effluent is
preferably dispersed into the ground through a soakaway system. Less acceptably,
the effluent may be discharged to a ditch or watercourse but its quality is such
that this may give rise to odour problems or pollute the recipient.
Periodically, sludge is sucked from the septic tank by a purpose-built road
tanker and conveyed to a plant for further treatment and safe disposal.
For larger premises in rural areas, such as hotels and
restaurants, a small treatment plant, often factory-built, will be installed.
These plants use the same basic treatment processes as municipal wastewater
plants. However, arrangements are normally made to convey the sludges that they
produce to a municipal plant for treatment and safe disposal.
2.5.2 When is the transition made from individual on-site
treatment to the construction of a sewer system to transmit sewage to communal
A sewer system and communal treatment plant are constructed when
it is either cheaper to do so than construct and operate individual on-site
facilities, or when the ground on which the development is situated is
insufficiently permeable to absorb the discharges from septic tanks. Under these
circumstances, unsanitary conditions and odour nuisance may result from this
practice or the quality of the groundwater may be reduced to an unacceptable
2.5.3 Do rural wastewater treatment plants for small
populations create particular problems?
Ideally, a wastewater treatment plant would be served by a short
sewer system, receive a constant flow rate of wastewater and be of a size to
justify a full compliment of technical and support staff working on-site. Sewer
systems and small treatment plants serving scattered rural populations and
villages do not generally satisfy these criteria and so the problems created
must be taken into account when planning their design and operation, viz.:
· long lengths of
sewer and pumping mains serving small populations have long retention times and
wastewater can become septic, creating offensive odours and difficulties in
· the smaller the population
served, the more variable is the wastewater flow rate and the pollution load
arriving at the treatment works throughout the day and treatment units must be
designed to take this into account;
· it is often difficult to
allocate operational staff exclusively to a small treatment works; if there are
a number of such works in an area, the formation of a mobile operations and
maintenance team might be justified, otherwise regular visits should be made by
the staff from a larger works, suitably trained in the operational problems of