Characteristics of wastewaters

Municipal wastewater is mainly comprised of water (99.9%) together with relatively small concentrations of suspended and dissolved organic and inorganic solids. Among the organic substances present in sewage are carbohydrates, lignin, fats, soaps, synthetic detergents, proteins and their decomposition products, as well as various natural and synthetic organic chemicals from the process industries. Table 1 shows the levels of the major constituents of strong, medium and weak domestic wastewaters. In arid and semi-arid countries, water use is often fairly low and sewage tends to be very strong,  where water consumption is 90 l/d per person. 

Table 1: MAJOR CONSTITUENTS OF TYPICAL DOMESTIC WASTEWATER

Constituent

Concentration, mg/l

	Strong	Medium	Weak
Total solids	1200	700	350
Dissolved solids (TDS)1	850	500	250
Suspended solids	350	200	100
Nitrogen (as N)	85	40	20
Phosphorus (as P)	20	10	6
Chloride1	100	50	30
Alkalinity (as CaCO3)	200	100	50
Grease	150	100	50
BOD52	300	200	100

¹ The amounts of TDS and chloride should be increased by the concentrations of these constituents in the carriage water.
² BOD₅ is the biochemical oxygen demand at 20°C over 5 days and is a measure of the biodegradable organic matter in the wastewater.

Physical

Wastewater has physical characteristics such as temperature, solids, odour and colour. In plumbing work the temperature and type of solids in the wastewater are important considerations. Wastewater at high temperature will affect some piping materials and treatment units such as septic tanks. You may have to consider the use of an arrestor to pre-treat the wastewater.

Chemical

Wastewater contains chemicals such as nitrogen, phosphorus and levels of dissolved oxygen as well as others that may affect its composition and pH rating. Highly acidic or alkaline wastewater is probably trade waste and will require pre-treatment before discharge to the sewer.