Sewage is the wastewater and waste matter carried in the water discharges from homes, offices, shops and factories. Sewage consists of approximately 99.9% water and contains material like grit, grease, paper, plastic and human waste. Wastewater discharged from industrial activities, commonly called trade waste, may contain 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 rain water runoff from surfaces such as roofs and roads, along with drainage from our gardens and open lands.
Sewage from properties connected to a sewerage system flows to a treatment plant where it is treated to a specified quality level.
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 and 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 Tasmanian Plumbing Code and requires Council approval. See the Workplace Standards Tasmania 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.
Treatment plants vary in size and complexity. 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, with the required level of technological sophistication increasing to achieve higher levels of treatment.
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 dissolved or suspended organic matter to a form that reduces its environmental impact if discharged. Disinfection by means of chlorination, ozonisation or UV radiation, is also often considered to be part of the secondary treatment step.
In 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 by way of ocean outfall, where nutrients are dispersed rapidly. Effluent reuse also usually requires treatment to secondary level, provided that the specified disinfection limits can reliably be achieved.
Tertiary treatment is becoming a standard requirement for effluent discharged to waterways which 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.
Wastewater treatment produces two types of material: treated effluent and concentrated solids that have been removed from the wastewater. The solids have traditionally been known as 'sewage sludge' but within the recycling industry, the term 'biosolids' has become widely accepted.
It is important to realise that when more efficient wastewater treatment processes are employed, the amount of biosolids produced increases. For example, the removal of phosphorus from wastewater is often achieved by means of chemical precipitation with metallic salts, with the resulting flocs settling out in the sludge. Therefore, the sludge becomes more voluminous and problematic to deal with, while the quality of the effluent is improved.