Sewage Sludge Residual Process

The links below represent the flow of sewage sludge residuals from the wastewater treatment processes and then thickened, stabilized, thickened further, stored, and transported by barge for further treatment and dewatering at the Passaic Valley Sewerage Commissioners (PVSC) treatment works in Newark, New Jersey. Click on the respective link to learn about each step of the BCUA’s sewage sludge residual process.

 Sludge Thickening

Primary and secondary sludge thickening is beneficial to the anaerobic digestion process because it reduces biomass volume tank size and heating requirements. Untreated sludge is a suspension of solids in water. One of the primary goals of sludge treatment is the further separation of solids from water to make treatment and disposal more effective and efficient. Sludge thickening to increase the solids content of the sludge is the first step of the sludge treatment process. The Little Ferry WPCF utilizes four gravity thickeners and three gravity belt thickeners to thicken sludge. The gravity thickening process consists of pumping primary and secondary sludge into a gravity thickening tank where the sludge is mixed and agitated gently by a rotating mechanism. The solids tend to settle to the bottom of the tank where they are pumped into the anaerobic digesters. The thickening tank supernatant is pumped to the headworks of the treatment plant for treatment. Sludge is also screened and degritted prior to distribution in gravity thickening tanks to prevent pipe clogs, premature wear, and accumulation of grit in digester tanks.

The gravity belt thickeners are used primarily to thicken secondary sludge that is wasted from the system. Secondary sludge is more difficult to thicken than primary sludge, especially during warm weather when the density of the secondary sludge is reduced. The gravity belt thickeners add thickening capacity during these critical periods.

Sludge thickening whether by gravity or gravity belt thickeners, relies on the addition of polymer to aid the process. Polymers act as flocculating agents, causing the particles of sludge to adhere and form larger particles, or flocs. The flocs are heavier than the smaller particles and settle more readily, thereby improving the efficiency of the sludge thickening process.

 Anaerobic Digestion & Reuse

Thickened sludge has a solids content of approximately 4-5% solids depending on season. While thickening increases the solids concentration of the sludge, the material still contains pathogens and putrescible organic matter. Anaerobic digestion is a means of reducing both the pathogen population and the volatile organic content of the sludge, thereby making the material more stable and easy to manage during ultimate disposal.

Thickened sludge is pumped from both the gravity thickeners and thickening gravity belt units into anaerobic digesters. The Little Ferry WPCF has five 80' diameter anaerobic digesters. During the digestion process, the sludge becomes the food source for anaerobic bacteria, which in the absence of oxygen consume the organic material in the sludge and produce methane gas as a byproduct of respiration. The methane gas rises to the surface of the tanks where it is collected from under the digester covers and used as an energy source for the four boilers which provide the heat for the entire wastewater treatment facility. The microorganisms, by releasing methane gas, reduce the volatile organic content of the sludge by approximately 50-60%.

The anaerobic digestion process must occur under carefully controlled conditions. The temperature must be maintained between 95 to 100 degrees Fahrenheit. This temperature range is the optimum temperature for the anaerobic bacteria that feed on the sludge, but tends to inactivate the pathogens in the sludge, thereby resulting in significant pathogen reduction. Some methane gas is recirculated through the tanks to keep the sludge completely mixed and in contact with the anaerobic bacteria. Other parameters such as the pH and ammonia concentration in the tanks must also be carefully monitored. Since the process depends on the absence of oxygen, the digester covers are designed to float on the surface of the sludge, creating an airtight seal around the edge of the cover. The anaerobic sludge digestion process requires a holding time of 12-16 days. The anaerobic digestion process produces a sludge that once again has a solids content of approximately 3%. So much methane is produced during anaerobic digestion that the mass of the material is reduced considerably, thereby reducing the solids concentration. At this stage in the sludge treatment process, the material is relatively stable and free of pathogens.

Digested sludge from the anaerobic digesters is pumped by digested sludge transfer pumps to wet wells at the Digested Sludge Thickening/Dewatering (DST) Building. The sludge is then pumped by digested sludge feed pumps equipped with VFDs to combination GBT/BFP units. The digested sludge transfer speed is varied to maintain a constant level in the wet well. Polymer solution is injected into the digested sludge feed line for each GBT/BFP. In the sludge thickening mode of operation, only gravity thickening function of the GBT/BFP is utilized. The thickened digested sludge from each GBT/BFP is discharged into a sludge hopper connected to the thickened digested sludge pump. The thickened sludge pump, pumps the sludge to either the Sludge Storage Tanks or directly to a barge for disposal to Passaic Valley Sewage Commission (PVSC).
Digested sludge can also be dewatered. In this mode of operation, both gravity thickening and dewatering portions of the GBT/BFP are utilized. The dewatered sludge is discharged from GBT/BFP into screw conveyors which direct the sludge cake to roll-off containers. The roll-off containers are subsequently trucked offsite.
An in-line grinder is provided upstream of each digested sludge feed pump. The grinders macerate coarse solids to protect the GBT/BFP belts and extend belt life.
One dedicated digested sludge pump is provided for each GBT/BFP. The pumps are equipped with VFDs to handle varying digested sludge flow rates. A fifth pump is dedicated to the BFP.

One dedicated thickened digested pump is provided for each GBT/BFP. The pumps are equipped with VFDs to handle varying thickened digested sludge flow rates and to maintain a sludge hopper set point.

In the sludge dewatering mode, the dewatered sludge cake is conveyed from GBT/BFP units by screw conveyors to 20-cubic yard roll-off containers for disposal. One common longitudinal conveyor, which receives sludge cake from all GBT/BFP, transports the cake to 3 cross-conveyors. Isolation gates are provided to allow the Operator to direct the sludge cake to the desired container and cross conveyor.

Two dry polymer mixing systems are provided within the DST Building. One unit is designed to handle current and future polymer requirements, with the second unit acting as a standby system. One dedicated polymer feed pump is provided for each GBT/BFP. The pumps are equipped with VFDs to handle varying feed rates. Potable city water connections to the polymer feed pumps discharge piping are provided to allow further dilution of the polymer solution if required. Each water supply connection includes manual valves, a pressure regulating valve and a rotameter to allow for manual control of the dilution flow rate.

A potassium permanganate feed system is provided to allow injection of potassium permanganate solution into the digested sludge feed upstream of the DST wet well. Injection of potassium permanganate reduces sulfide concentration in the sludge and improves thickening/dewatering characteristics of the digested sludge feed. The filtrate and washwater collected from the GBT/BFP flows by gravity back to the head of the treatment plant.

To control odor, the exhaust air from the process area odor enclosure is passed through carbon adsorption vessels using odor supply fans.