The Treatment Process
Every day approximately 11 million gallons of wastewater flows from Nashua and Hudson into the Nashua Wastewater Treatment Facility. Wastewater is sent from homes, businesses, and industries into Nashua’s sewer/collection system. At the facility, the wastewater is channeled through several different treatment processes to remove pollutants before being discharged into the Merrimack River.
In preliminary treatment, two mechanical bar screens are used to remove large solids and other coarse debris that might damage downstream pumps and other equipment. The flow then proceeds to aerated grit chambers where the grit and sand is allowed to settle. This material is washed and loaded into trucks for disposal at the city’s landfill.
Nashua Wastewater Treatment Facility
Primary Sedimentation Tanks
In the next phase, known as primary treatment, the wastewater flows to primary sedimentation tanks where settleable and floatable solids separate out naturally. In this step, approximately 50% of the solids and 30% of the organic material are removed from the wastewater in the form of primary sludge.
In secondary treatment, the wastewater flows into four large aeration tanks where bacteria and other simple organisms grow and multiply, digesting the organic matter as food. This mass of organisms and solids is then allowed to separate from wastewater in the secondary clarifiers, resulting in a purified liquid known as effluent. The separated excess biological organisms are called secondary sludge. At the end of the secondary process, more than 90% of the organic materials and solids have been removed from the wastewater.
In the final step, Sodium hypochlorite a strong bleach is then added to the effluent to kill any remaining harmful bacteria. The disinfected effluent is then completely neutralized to remove excess hypochlorite in order to protect aquatic wildlife in the river.
The sludges created in primary and secondary treatment processes are thickened to remove the majority of the water they contain. Tanks called gravity thickeners are used to thicken primary sludge, while mechanical devices known as belt thickeners are used to thicken the secondary sludge. The thickened sludges are then sent to the new anaerobic digester complex. In the 1.3 million gallon egg shaped primary digester, the sludge is held for 20 days. Here the solids are further broken down into carbon dioxide, water and methane gas. The methane is sent to a generator to produce electricity and to a boiler to produce heat for the digestion process. Anaerobic digestion eliminates between 40% to 50% of the sludge that needs to be disposed of and this amounts to a reduction from 19,000 tons/year pre-digestion to less than 10,000 tons/year with digestion on line.
After leaving the digester complex, the digested sludge is sent to three inclined screw presses for dewatering, creating a material similar in consistency to damp soil. Presently the dewatered sludge is discharged to conveyors, loaded onto trucks, and hauled by an outside contractor to farms as a soil enhancer.