Wastewater

THE WASTEWATER PROCESS FOR THE BOROUGH OF SHIPPENSBURG

The general service area of the Shippensburg Wastewater Treatment Plant includes the Borough of Shippensburg and several adjacent municipalities. These adjacent municipalities are members of the Cumberland-Franklin Joint Municipal Authority which, by agreement will, discharge wastewater from the municipalities to the Shippensburg Borough Authority’s facility. The adjacent municipalities include: Orrstown Borough and portions of Southampton Township (Franklin County), Southampton Township (Cumberland County), and Shippensburg Township. The service area is drained to the Conodoguinet Creek by Middle Spring and Mud Run Creeks.

The general service area is largely residential, with several commercial and industrial areas. Manufacturing is the largest industry group; however, their contribution to wastewater flow is less than 10 percent of the treatment plant design capacity. The single largest wastewater contributor, in the service area, is Shippensburg University, located in Shippensburg Township.

The wastewater treatment plant was designed to serve a projected 1995 sewer service area population of 21,000. Included in this figure is a Borough population of 8,500, a Joint Authority population of 8,000, and a university population of 4,500.

The selected process utilizes a combination of biological, physical, and chemical treatment processes. Biological treatment incorporates trickling filtration and activated sludge processes for BOD and suspended solids removals and nitrification. Physical treatment incorporates gravity settling and granular-media filtration for SS and BOD removals. Chemical treatment incorporates alum (aluminum sulfate) and polymer addition for phosphorus removal, lime addition for pH adjustment, and chlorine addition for disinfection. Together, these processes are expected to produce an effluent meeting the requirements of the NPDES (National Pollutant Discharge Elimination System) permit. Solids generated in these processes are mechanically thickened, anaerobically digested, and de-watered on a belt-filter press prior to disposal at an approved site.

FACILITIES DESCRIPTION

The Shippensburg Borough Authority’s facilities include approximately 23 miles of collection and conveyance sewers, one pumping station, two metering chambers, and the treatment plant.

Construction of the original treatment plant and sewer system were completed in 1954. Additions to the plant to accommodate increased volumes from Shippensburg University were completed in 1968. Additions and alterations to the plant were constructed during 1978-1981 to enable the plant to produce an effluent that met the limitations imposed by state and federal governments.

Wastewater enters the treatment plant at the plant pumping station. Here, the flow is measured by a parshall flume while rags and other debris in the wastewater are shredded by a comminutor. Comminuted wastewater passes into the wet well. Raw wastewater pumps lift the wastewater from the wet well to the primary distribution box. The function of this chamber is to evenly distribute the flow into the two primary clarifiers. This chamber also gives the Borough the ability to add chemicals to precipitate phosphorus compounds in the primary clarifiers.

As the sewage enters the primary clarifiers the fluids’ velocity is reduced to a point where granular materials, like sand and gravel, may settle to the bottom of the tank by means of gravity. Smaller, flocculent materials may remain suspended until natural flocculation or aggregation of these particles form larger particles that settle like sand and gravel. This is the process of plain sedimentation. If chemicals are added to increase the likelihood of flocculation, the process is called chemical precipitation.

These settling or sedimentation tanks may reduce the amount of suspended material from 45% to 80 % and reduce the BOD (Biochemical Oxygen Demand – a measure of the strength of the sewage) by 25% to 40%.

Effluent from the primary clarifiers is discharged to another distribution chamber where screw pumps lift the wastewater to the first-stage biological treatment process. The first-stage process (trickling filters) is designed to remove large amounts of carbonaceous biochemical oxygen demand (CBOD) present in the wastewater before it is discharged to the second-stage biological treatment process (activated sludge). The first-stage process utilizes trickling filters is followed by alum and polymer mixing for additional phosphorus and suspended solids removal, secondary clarification, and recirculating of wastewater to affect treatment.

The trickling filters rely on bacteria and other microorganisms to grow where sewage flows over a suitable surface. This zoogleal film is teeming with these tiny organisms that break down complex organic materials into simple, more stable substances.

The rotating arms of the trickling filters apply sewage flows to a suitable medium (limestone rocks in our case) where these organisms reside. Because the destruction of organic materials occurs up to three times more rapidly in aerobic conditions the trickling filter has been designed to apply flows to the surface media on an intermittent basis. Alternating contact with the sewage followed by contact with the surrounding atmosphere is the optimum condition for continued growth of the microorganisms as well as the continued reductions in the strength of the sewage.

The sewage is applied to the trickling filter media, makes contact with the zoogleal film, drains down through the media and is collected in a tank under-drain system. This trickling filter effluent flows by gravity to the secondary alum mix tank to receive a dose of chemicals before entering the secondary clarifiers.

The addition of polymers and alum (aluminum sulfate) allows for the chemical precipitation of phosphorus compounds in the secondary clarifiers. Polymers act like a glue to coat individual suspended particles that will not normally settle out on their own. When one “sticky” particle bumps into another, the two will stick together. As these “clumps” grow in size they will eventually grow too big and too dense to continue to suspend in the water. At that point the floc will settle to the bottom of the tank much like the sand and gravel did in the primary clarifiers. Remember that when chemicals are added to increase the likelihood of flocculation, the process is called chemical precipitation.

Through this application of chemicals, the suspended material levels and the particulate and dissolved phosphorus compound levels are further reduced. With each step in the treatment process the sewage moves to the next process a little cleaner than before.

The second-stage biological treatment process (activated sludge) is designed to remove ammonia nitrogen from the wastewater. To affect efficient removal, lime is mixed with the wastewater to maintain the optimum pH for treatment. In the last process, secondary clarification, polymers and alum were added to chemically precipitate dissolved compounds and particulate materials from the wastewater. The addition of these chemicals lowered the pH of the sewage to a point below what is considered acceptable by the microorganisms in the second-stage biological treatment process.

In this treatment stage, the pH corrected wastewater is brought into contact with a pre-existing biological floc contained in the aerated tanks. The biological activity of the organisms convert the organics in the sewage into cell tissues, water, and carbon dioxide. The prime function of this treatment phase is to reduce the ammonia-nitrogen concentration and to reduce the carbonaceous oxygen demand of the wastewater.

The floc mass consists of millions of organisms including bacteria, fungi, yeast, protozoa, and worms. The organisms grow by taking food from the incoming wastewater. It is through this biological processing and uptake that the pollutant levels are reduced. Mixing of the biological floc with the incoming wastewater also promotes collisions that will produce larger floc masses. Even inorganic materials may be found in these floc masses. As these floc masses settle to the bottom of a clarifier, and a further reduction of pollutant levels may be expected.

Nitrification removes ammonia nitrogen, by converting it to nitrite nitrogen which can be further converted to nitrate nitrogen. Nitrogen in this form is nontoxic to fish and reduces the oxygen demand in the receiving stream.

After the flow has passed the length of the tank it is discharged to the final clarifiers. In these clarifiers plain sedimentation occurs. The sludge which collects in these tank bottoms contains large concentrations of the microorganisms needed in the nitrification tanks. Most of this sludge is recycled back to the head of the nitrification tanks to maintain an appropriate level of microorganisms. Excess sludge is removed from the system and sent to the anaerobic digesters for further reduction and stabilization.

The settled effluent from the final clarifiers flows to the dual media filters for further (tertiary) treatment. The granular-media filters provide additional BOD5 and suspended solids removal by filtering the flows through layers of sand and coal. Filter effluent flows to the chlorine contact tanks for disinfection with chlorine and this chlorinated effluent is discharged to Middle Spring Creek.

Sludge and scum from the primary clarifiers, secondary clarifiers, and final clarifiers is pumped to the sludge thickener. Thickened sludge and scum are pumped to the anaerobic digesters for solids stabilization. The sludge produced in these various clarifiers have different settling characteristics and must be mechanically thickened before being pumped to the digester system. Primary clarifier sludge may settle well and be very thick while secondary and clarifier sludge are much thinner and rather light and fluffy.

The thickening process allows these different sludges to be mixed together and thickened. Once thickened to a textbook level of 5% – 8% solids, this sludge can be pumped to the primary digester. It is here that the thickness of the incoming sludge becomes important. In the Primary Digester, the sludge is heated to an optimum 95 degrees Fahrenheit. This temperature has been identified as being the best temperature to maintain the anaerobic (no oxygen)organisms that breakdown and stabilize the sludge in an appropriately short time. So less water and more thickened sludge pumped to the digester is more efficient for the sludge operation and less expensive than heating a lot of water that doesn’t need treatment.

The primary digester is always full of fluid and as more sludge is pumped into it an equal volume is displaced flowing into the secondary digester. This digester is the storage facility for sludge generated in the various plant processes. From this point the sludge can be processed and removed from the plant.

Digested sludge from the secondary digester is pumped to a belt-filter press for de-watering. Polymer is added to improve de-watering efficiency. De-watered sludge is trucked to an approved disposal site and/or stored on site for future disposal.

The Wastewater Treatment Plant is staffed by seven full time employees. Daily operating hours are from 7:00 A.M. until 3:00 P.M. Pump timers and other electronic wizardry allow for this one shift operation.