Wastewater Treatment Processes

 Nassau County Department of Public Works
Summary of Wastewater Treatment Processes

 

 

Preliminary Treatment

The influent wastewater enters the screening chamber and passes through mechanically cleaned bar screens. All the debris removed from the bar screens is discharged into containers. The debris is subsequently compacted and dewatered and deposited into removable containers for offsite disposal.

The wastewater is then pumped to grit chambers where the removal of grit, consisting of sand, stones, cinders, and other heavy inorganic material, takes place. The early removal of this material prevents abrasive action on mechanical equipment such as pump impellers, packing glands, and collector mechanisms.  In addition, it reduces hazards of clogging in Plant units such as pipelines, tanks, and hoppers and also simplifies sludge handling and disposal.

 

Primary Treatment

After the grit chambers, the wastewater flows to the primary sedimentation tanks. The primary tanks are designed to remove about thirty percent of the BOD5 and about forty‑five to fifty‑five percent of the suspended solids.  The solid particles suspended in wastewater are carried forward by the velocity and mixing action of the flow.  In the primary sedimentation tanks, the velocity and mixing action is greatly reduced.  Solids that will settle under these conditions are collected at the bottom of the tank, where they form sludge which is pumped to the anaerobic digesters.

 

Secondary Treatment

The effluent from the primary tanks containing the non-settleable solids and soluble organic material flows to the aeration tanks. In the aeration tanks, the suspended solids and dissolved contaminants are reduced by the biological growth in the liquid.  Microorganisms, which reduce the BOD5 and other contaminants in the liquid, form a floc in the presence of dissolved oxygen.  Oxygen is provided in the air supplied by the blowers.  The floc is also kept in suspension by the movement of the circulating air in the aerators.

The effluent from the aeration tanks enters the final settling tanks where the floc that was formed in the aerators is settled out. Most of the sludge from the final settling tanks is returned to the aeration tanks to maintain the biological growth.  This returned sludge, containing the active living organisms settled from the aeration tank effluent, is termed return activated sludge or RAS.  Excess sludge is “wasted” (waste activated sludge or WAS) to the sludge thickener. The secondary treatment process can operate as either conventional activated sludge or step aeration activated sludge.

The effluent from the final settling tanks flows through traveling water screens with 1/8″ square openings to remove floatable material which has not settled in the final settling tanks. Screenings are pumped to the sludge thickener.

 

Disinfection

The Plants use sodium hypochlorite for effluent disinfection, influent odor control, and prevention of sludge bulking. The chlorine destroys the pathogenic organisms in the effluent before it is discharged to the receiving water.

After being disinfected, the Plants effluent is discharged through the outfall pipe into the receiving waters. At low tides, the plant effluent flows by gravity through the outfall pipe.  At high tides, the effluent is pumped through the outfall pipe.

 

Sludge Thickening

Waste activated sludge collected in the final settling tanks is pumped to the sludge thickening facility, for concentration, before pumping to anaerobic digesters. Thickening is accomplished by gravity belt thickeners.  The thickened sludge is pumped to anaerobic digesters.

 

Sludge Digestion

The volatile solids in the sludge are reduced fifty to sixty percent in the anaerobic digesters. This is accomplished by acid forming and methane forming bacteria. The digesters are operated in the mesophilic range (85 to 100 °F).  Methane gas is collected from the digesters and used as fuel for the engines to power the plant. The methane provides about one third of the Plant’s power needs.  Digested sludge is pumped to the sludge dewatering facility.

 

Sludge Dewatering

The anaerobically digested sludge is received at the dewatering building at about 1.5-2.0% solids content. It is then sent through belt dewatering presses which remove much of the water. The resulting sludge cake has a solids content of about 20%. The dewatered sludge cake is transported by truck out of state to a facility for further processing into compost so that it can be beneficially re-used.

 

Odor Control

Due to the proximity of the area residents and two schools, the Plant has constructed and maintains odor control scrubbers at all of the process buildings. All of the air exhausted from these buildings is passed through packed tower scrubbers to remove odor causing compounds prior to its release.

 

Power Generation

The Plants generate all of their power on-site. We operate and maintain engine driven generators. The engines which run these generators can run on fuel oil, natural gas or methane from the digesters.