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4.2 Off-site wastewater treatment systems

Off-site treatment is the treatment of wastewater that has been conveyed using a sewerage system (Section 2 (3)). Activated sludge treatment is now considered the conventional means of large-scale off-site treatment of sewage, and is described first. Trickling filtration is an alternative that was developed earlier than the activated sludge process, and this is described next. There have traditionally been other more simple, but as effective methods of treating sewage. These include the use of ponds or lagoons, land based treatment (sewage farming), and aquaculture. The first two are described in this section, while aquaculture is described under wastewater reuse (Section 2 (6)), because wastewater is generally treated first prior to aquaculture.

Several general principles common to treatment systems will be discussed first. The main aim of treatment is to reduce biochemical oxygen demand (BOD) and suspended solids (SS) to acceptable levels. This is achieved by removing solids and aerating the wastewater to satisfy the oxygen demand of the wastewater. The different treatment systems achieve the removal of solids and in providing oxygen in different ways. It should be noted that if the systems are properly designed, constructed, operated and maintained, they should all achieve the required standard of treatment. The latter is generally a reduction of BOD to less than 20 mg/L, and SS to less than 30 mg/L.

Nutrients (nitrogen and phosphorus) may need removal if the wastewater is discharged to water environments sensitive to enrichment by nutrients. The North America and Western Europe Regional Overviews contain details of methods for removing nutrients, because nutrients have been found to be a problem in many receiving waters. Heavy metals and other pollutants are not generally a problem unless the sewerage system receives industrial discharges. In this case treatment of industrial wastes prior to discharge to the sewerage system is the solution to this problem.

Removal of SS and BOD produces sludge, and the sludge has to be treated prior to reuse or disposal (Section 2 (5)). Anaerobic treatment has recently been suggested for wastewater. The main reason for the use of an anaerobic process is the recovery of energy (in the form of methane) from the wastewater (see Section 2 (2.3)) for explanation of the anaerobic process). The upflow anaerobic sludge blanket process is described at the end of this section.

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