Oil Skimmer Model 6V Removes Fat from Wastewater
PROBLEM: The Pennsylvania Department of Environmental Resources has issued Orders that set new stringent discharge standards.
College Hill Poultry, Inc. in Fredericksburg , Pa. , processes 25,000 birds a day. Operators use water for scalding, product preparation, cooling whole birds and parts, offal rendering, and sanitation.
The 55,000-gpd wastewater contains significant amounts of soluble and suspended organic matter. In the past, wastewater was treated to meet secondary treatment standards by use of an aerated lagoon treatment system.
To meet new effluent requirements, an engineering firm was contracted to study the problem. Upon their recommendations, the poultry processor initiated a water-conservation program based on the theory that water conserved is water that does not require treatment. The water conservation program resulted in an increase in pollutant concentrations; however, the reduction in water usage produced a decrease in total pounds of matter entering the effluent. With water usage at the lowest allowable by USDA, efforts were next directed at finding ways to treat the spent process water with regard to state and federal environmental regulations, and capital, operating, maintenance, and energy costs.
SOLUTION: College Hill installed a pretreatment system consisting of a rotating mechanical screen and a Model 6V oil skimmer from Oil Skimmers Inc. for byproducts recovery. With a unique anti-clogging design, the 6V oil skimmer has a polyurethane tube that attracts and collects the waste oil as it floats on top of wastewater. The tube passes through a scraper system to remove the oil, which flows by gravity into a containment drum. They also installed a flocculation tank, and a dissolved-air flotation (DAF) tank; and a secondary treatment system employing a West-German design, counter-current low-load aeration process to organically polish and nitrify the effluent before discharging into the receiving stream.
First step in the pretreatment process is screening to remove gross solids. The velocity of the flowing waste water through the rotating wire-wedge screen drum forcefully holds the solids against the screen. The trapped solids ride over the top of the screen as the drum revolves. Larger/heavier solids fall freely as they start their downward path, while smaller solids are removed by a doctor blade that rides in contact with the drum. All screenings recovered are collected in a small hopper, which is connected to a dry-offal, vacuum-transport conveyor system. The conveyor moves the screenings to the rendering building, where they are processed for use as a proteinaceous animal feed.
Located beneath the rotating mechanical screen is a small-volume wet well used to equalize the flow into the hydraulically sensitive DAF unit. A continuous-loop Model 6V oil skimmer automatically collects and removes around 100 gallons of oil per hour of floating grease from the wastewater contained in the wet well. Skimmed grease is collected for rendering in the same hopper used to collect and transport the screenings.
To enhance flocculation, a coagulant is added, followed by a pH adjustment to promote floc formation in the mechanical flocculator. Good rapid mixing of these chemicals is of the utmost importance to system performance. After 10 minutes of gentle agitation to enhance flocculation of minute grease globules and other coagulated particles, the flocculated wastewater continues its flow by gravity to the head of the DAF tank. Here, it is mixed with a portion of the DAF unit’s effluent, which has been pressurized to about 60-70 psig and aerated. When this recycled flow is depressurized to atmospheric pressure, fine air bubbles are formed.
As the combined flow enters the DAF tank, the fine air bubbles adhere to the flocculated particles – increasing their buoyancy and causing them to separate from the wastewater. The partially treated wastewater discharges over a weir and onto the secondary treatment system. Flocculated particles form a sludge blanket (float) on the surface of the water. Sludge is automatically skimmed off and transported to a hopper. A sludge pumping system moves that float from the hopper to an anaerobic sludge holding tank. The pretreatment system has reduced BOD 80%, SS and phosphate 90%, and Ammonia-Nitrogen 30%.
The secondary treatment system – countercurrent low-load aeration process – consists of a circular tank having an outer ring for aeration, and inner-circular tank for final clarification. The treatment system is equipped with blowers, rotating bridge with fine-diffused aerators, sludge-thickening pit, and return-sludge station.
After pretreatment, the wastewater enters the outer-ring aeration tank, where it comes in contact with the flocculated activated sludge. Here, it is aerated and agitated until the total organic pollution has been aerobically decomposed by microorganisms in the presence of oxygen.
The countercurrent low-load aeration process utilizes the principle of diffused aeration in horizontally moving streams. In vertical or conventional diffused-air systems, the ascending air bubbles rise to approximately the same place – creating a vertical updraft of the water towards the surface. Since the air bubbles rise quickly, detention time is short. Consequently, oxygen transfer rates are low.
With a horizontal current, the air bubbles are deflected by the laterally moving liquid – causing them to ascend slowly over a diagonal path to the surface. This longer detention time permits a greater oxygen transfer between the air and wastewater.
After sufficient aeration, the mixed liquor-activated sludge flows through a pipe into the clarifier tank. Sludge settles to the bottom of the tank, while the purified effluent passes the overflow weir, is chlorinated, and leaves the system.
The settled sludge in the clarifier tank is transferred to the return sludge station via a screw pump. Approximately 90% of the activated sludge is returned to the system, with the remainder being wasted to the sludge thickening pit; where it is allowed to decant for further thickening. From the pit, the sludge is pumped to storage tanks. Periodically, the sludge is hauled away in tank trucks and spread on agricultural land.
RESULTS: The pretreatment and secondary treatment system at College Hill Poultry have cut BOD 99.7%, SS 98.7%, Ammonia-Nitrogen 97.3%, and phosphate 95.1% bringing the company into compliance with the state’s stringent discharge standards. In addition, the grease and oil loads in the effluent have been reduced to 4.6 mg/l.
The unique equipment/treatment combination is recovering valuable byproducts for rendering, and is oxidizing the remaining organic pollutants into a totally stabilized sludge with a nearly complete nitrified effluent.