The sewage treatment plant primarily uses physical and not chemical methods to separate SOLIDS from water.

At the City's wastewater treatment plants, wastewater undergoes five major processes: preliminary treatment, primary treatment, secondary treatment, disinfection and finally, sludge treatment. Primary and secondary treatments remove about 85% to 95% of pollutants from the wastewater before the treated wastewater is disinfected and discharged into local waterways. Sludge, the byproduct of the treatment process, is digested for stabilization and is then dewatered for easier handling. The resulting material, known as biosolids, is then applied to land to improve vegetation or processed further as compost or fertilizer. (See illustration — “Diagram of Treatment Process” on page 16-17.)...

Primarily gravity is the tool of choice.

...The aerated wastewater then flows to the final settling tanks which are similar to the primary settling tanks. Here the heavy particles and other solids settle to the bottom as secondary sludge. Some of this sludge is re-circulated back to the aeration tanks as “seed” to stimulate the activated sludge process. The returned sludge contains millions of microorganisms that help maintain the right mix of bacteria and air in the tank and contribute to the removal of as many pollutants as possible....

...Even after primary and secondary treatment, diseasecausing organisms may remain in the treated wastewater. To disinfect and kill harmful organisms, the wastewater spends a minimum of 15-20 minutes
in chlorine-contact tanks mixing with sodium hypochlorite, the same chemical found in common household bleach. The treated wastewater, or effluent, is then released into local waterways.... Disinfection is an essential step because it protects the health of people who use
local beaches and enjoy other recreational activities on or near the water....

Ecological Characteristics

- Values for sodium hypochlorite:
  - Acute oral-bobwhite quail: LD50, > 2510 mg/kg
  - Acute dietary-mallard duck: LC50, > 5220 ppm
  - Acute dietary-bobwhite quail: LC50, > 5620 ppm
  - Acute fish-rainbow trout: LC50, 0.18-0.22 mg/l
  - Acute fish-bluegill sunfish: LC50, 0.44-0.79 mg/l
  - Acute invertebrate-daphnia: LC50, 0.033-0.048 mg/l

Lethal dose (LD50) is the amount of an ingested substance that kills 50 percent of a test sample. It is expressed in mg/kg, or milligrams of substance per kilogram of body weight. Common name. Toxin. Lethal doses.

- Formulations:  Liquid sodium hypochlorite solutions; solid (granular,
  powder, or tablet) calcium hypochlorite.

- Sodium and calcium hypochlorite single active ingredient products are
  eye irritants, but the potential for hazard from use of these products
  may be mitigated with appropriate precautionary labeling.

- The hypochlorites are the most widely used chemicals for disinfecting
  water supplies, are generally recognized as safe for use as post
  harvest fungicides on agricultural commodities, and are listed as
  sanitizers, for use as terminal sanitizing rinses of food handling
  equipment.  In the absence of significant long term dietary exposure
  to the hypochlorites from these patterns of use, there is no need for
  chronic or subchronic studies.  If lt comes to the attention of the
  Agency that hazards due to long term dietary exposure may be
  significant, then long term studies will be required.

The final steps for waste water treatment is to put "anaerobic bacteria" into the

sludge tanks to further breakdown any additional solids, then centrifuged.