United States has a vast System of collection sewers, pumping stations, and treatment plants. Sewers
collect wastewater from homes, businesses, and many industries, and deliver it to plants for treatment.
Most treatment plants are built to clean wastewater for discharge into streams or other receiving waters,
or for reuse.
Primary Stage – The solids are allowed to settle and be removed from wastewater.
– As sewage enters a plant for treatment, it flows through a screen, which removes large floating objects
such as rags and sticks that might clog pipes or damage equipment.
– It then passes into a grit chamber, where cinders, sand, and small stones settle to the bottom.
– Sewage still contains organic and inorganic matter along with other suspended solids. These solids are
minute particles that sink to the bottom where they form a mass of solids called raw primary biosolids
(sludge) which can be removed from the tanks by pumping. The sludge is used as fertilizer or disposed
of in a landfill or incinerated.
The Secondary stage uses biological processes to further purify wastewater.
– The secondary stage of treatment removes about 85% of the organic matter in sewage by making use
of the bacteria in it. The principal secondary treatment techniques used are the trickling filter (a bed of
stones from 3-6 feet deep through which sewage passes) or (interlocking pieces of corrugated plastic or
other synthetic media) which then flows to another sedimentation tank. The trend today is towards the
use of the activated sludge process which speeds up the work of the bacteria by bringing air and sludge
heavily laden with bacteria into close contact with sewage. It is pumped into an aeration tank, where it is
mixed with air and sludge loaded with bacteria and allowed to remain for several hours breaking down the
organic matter into harmless by-products (primary effluent, sludge basin, air compressor, clarifier, recycle
pump, return sludge, back to clarifier, then secondary effluent, discharge to river or land application)
Applications Pumping Systems, Settling Tanks, Aeration Tank, Final Clarifiers, Chlorine Disinfection,
Ultraviolet Disinfection, Filtration, Water reuse, Solids Thickening, Digesters, Solids, Dewatering (Belt
Presses), Biosolids Recycling, Wastewater Quality Monitoring, Chemical Testing, Chemical Feed
Systems, Wastewater Evaporators, Biofiltration Systems, Bioreactor Systems, Skimming Collection
– Aerobic ponds – Shallow, light penetrates to bottom, active algal photosynthesis, Organic matter
converted to CO2, NO3, HSO4, HPO4
– Facultative ponds – Pond 1 – 2.5m deep – not easily subject to upsets due to fluctuations in Q, loading
– Anaerobic ponds – Primarily used as a pretreatment process for high strength, high temperature wastes.
Can handle much higher loadings…Stage 1 – Acid fermentation (Organics – Organic acids). Stage 2
– Methane fermentation Organic Acids to CH4 and CO2.