Municipal waste water, including sewage, is treated in a multistep process before the treated water is released into the environment.
Waste Teatment Overview
The treatment process gives recycled water and biosolids. Biosolids are predominantly water and organic material which come from the micro-organisms used in sewage treatment. They also contain:
macronutrients (nitrogen, phoshorus, potassium and sulphur)
micronutrients (copper, zinc, calcium, magnesium, iron, boron, molybdenum and manganese).
Biosolids may also contain traces of synthetic organic compounds and metal compounds, including arsenic, cadmium, chromium, lead, mercury, nickel and selenium. They contain the nutrient-rich organic materials derived from sewage solids (sewage sludge).
The average household's sewage leads to the production of between 0.5 and 1 kilogram of biosolids (dry weight) per week.
In some communities, the biosolids can be used as fertilizer.
Primary waste water treatment is the physical or chemically enhanced settling of suspended particles. It includes:
Aeration of the waste to remove odors and to oxidize Fe(II) to Fe(III). Many compounds with intense odors, such as sulfides and thiols, can be oxidized in air to compounds that don't have a bad smell.
The addition of Al2(SO4)3, Fe2(SO4)3, and Ca(OH)2. These materials precipitate as a flocculant material that traps the suspended particles in the waste water.
The waste goes into a tank where the undissolved solids fall to the bottom.
Primary treatment removes only one-third of the BOD and virtually none of the dissolved minerals. This doesn't remove soluble materials or toxic chemicals. The water retains a high BOD.
The clarified waste water is combined with aerobic heterotrophic bacteria, fungi and protozoa. These microorganisms break down the dissolved organics or incorporate them into new microbial cells (biomass) which then settle out of suspension, together with undigested material, as "sludge." The system must be well aerated to promote the growth of the aerobic organisms. Large pumps keep dissolved oxygen levels high. The sewage is continually circulated through trickle filters housed in "biotowers" or "activated growth units" where redwood slats are stacked and act as substrates for the attachment and growth of the microorganisms. The liquid sewage is sprayed over the slats to enhance aerobic conditions, and the organic compounds are broken down as the sewage trickles past the microbes growing on the slats. The water portion of the sewage runs through the tower, reenters the aeration tank, and is recirculated back through the towers for further clarification. As the microbial populations increase, foam appears on the surface and cells begin to clump together and settle. Some of the activated sludge is used to inoculate incoming effluent from the primary tanks.
Following treatment in the biotowers, the effluent flows into post-secondary sedimentation tanks where it is allowed to settle once again. The surface of the tank is slowly swept by a skimmer to remove surface growth on residual organics.
Tertiary treatment includes a range of processes that improve the water quality of the effluent before it is released into a lake, river, or into the ground. Not all sewage treatment plants use any type of tertiary treatment.