Sewage is the wastewater and waste matter carried in discharges from homes, offices, shops and factories. Sewage usually contains material like grit, grease, paper, plastic and human waste. Wastewater discharged from industrial activities may also contain other contaminants such as heavy metals and organic pollutants.
Sewage should not be confused with sewerage, the system of pipes through which sewage flows. Sewerage in turn should not be confused with stormwater drains, the system that collects rainwater runoff from surfaces such as roofs, roads, gardens and other open areas.
If a sewerage system is not available, wastewater will normally be treated on-site by means of a septic tank or a package treatment plant. The resulting effluent is usually discharged to in-ground absorption trenches. Any on-site treatment system needs to be designed and constructed in accordance with the Building Act 2016 and the associated Building Regulations 2016. It also requires Council approval. See the Consumer, Building and Occupational Services (CBOS) website for
accredited on-site disposal systems.
Once constructed, good management of such systems is important to prevent environmental impacts such as nuisance odours or the generation of leachate and run-off.
Sewage from properties connected to a sewerage system flows to a treatment plant where it is treated to remove contaminants. Treatment plants vary in size, complexity and treatment capacity. Small treatment plants are mostly based on lagoon systems, while larger population centres are often serviced by more complex systems.
Sewage treatment processes can be divided up into several basic steps:
Primary treatment involves screening the solids from the water and allowing a proportion of the suspended solids and organic matter to settle from the wastewater.
Secondary treatment takes primary-treated effluent and, with the aid of biological processes, breaks down a further proportion of the organic matter to a form that is less detrimental to the environment if discharged. Disinfection by means of chlorination, ozonation or UV radiation, may also be part of the secondary treatment step.
tertiary treatment, the secondary-treated effluent is further processed using various techniques including flocculation, coagulation, clarification and filtration. The main aim is to remove nutrients such as nitrogen and phosphorus and further reduce the small amount of organic material and any remaining harmful micro-organisms in the secondary treated effluent.
Secondary treatment is usually sufficient for effluent discharged to the ocean, where nutrients are dispersed rapidly. Secondary treatment is also usually enough for effluent to be safely reused for a number of land-based purposes, provided that disinfection is reliable.
Tertiary treatment is becoming a standard requirement for effluent discharged to waterways that are sensitive to nutrient enrichment, such as small inland watercourses or poorly flushed bays.
To make treated effluent suitable for potable (drinking water) reuse, any contaminants remaining after tertiary treatment must be removed through additional advanced treatment techniques such as ultrafiltration.
By-products of sewage treatment
Wastewater treatment produces two types of material: treated effluent and concentrated solids that have been removed from the wastewater. These solids are known as sewage sludge or, increasingly, biosolids.
When wastewater is more highly treated, more solids are removed, and the amount of biosolids increases. For example, phosphorus is often removed from wastewater using chemical precipitation with metallic salts to settle phosphorus flocs out into the sludge. Therefore, while the quality of the effluent is improved, the sludge becomes more voluminous and problematic to deal with. It is important to find the right balance between effluent quality and sludge production.