Waste Water Collection

In a centralised sewer system, no treatment happens at the individual homes or businesses. Collection lines carry raw wastewater to a treatment plant, where the wastes are biologically digested; the solids are incinerated, buried, or used as fertilizer; and the liquid portion is further treated and eventually discharged, usually to a body of water.

In a grinder sewer system, each home has a grinder pump that breaks up solid wastes and propels them to the central facility through a smaller diameter pipe than a conventional sewer system requires. This reduces the cost of building the collection network, as deep excavations and lift stations are not required. However, all waste treatment still must take place at the central facility.

With conventional septic systems, homes and businesses have no connection to the municipal sewer system. Each property discharges its wastewater into its own septic tank, where bacteria digest wastes. Solids and scum remain in the tank and are pumped out every few years for further processing and disposal at a central facility. The liquid effluent flows out into a drain field or filter bed on the same property, where natural processes finish the job of removing pollutants, and the purified effluent eventually returns to the groundwater or to surface waters.

An effluent sewer or cluster system combines elements of centralized sewer systems and septic systems. Each home or business has an interceptor tank for primary treatment, where bacteria digest the wastes. Instead of being discharged to a drain field at the site, the liquid effluent is filtered and conveyed to a collection line, which transports it offsite for further treatment and discharge to the soil at a nearby location that serves the whole neighbourhood or subdivision. Because the liquid effluent has already been partially broken down and filtered in each home’s interceptor tank, it contains a minimum of solids and needs only to be trickled through a sand or synthetic filter bed for further purification before it is discharged to groundwater or surface waters. Solids remain in each home’s tank, decomposing and reducing in volume, and any residual sludge is periodically pumped out and disposed of at the municipal facility.

These systems are called effluent sewer systems because only liquid effluent, not solids, is conveyed through the sewer system, or cluster systems because each filter facility serves a cluster of homes and businesses. Depending on how the effluent is conveyed to the offsite filter bed, they may be called septic tank effluent pump (STEP) or septic tank effluent gravity (STEG) systems.