Wastewater Treatment Process
Activated Sludge Process
Offline Storage Tanks
The Wastewater Treatment plant employs the following major process units to achieve desired effluent quality levels:
- Aerated Grit Chamber
- In-Line Equalization Basin
- Off-Line Stormwater Basins
- Activated Sludge Process
- Phostrip System
- Nitrification Process
- Denitrification Process
- Gravity Thickeners
All the influent wastewater entering the Treatment Plant passes through a mechanically cleaned bar screen. The bar screen is made of parallel steel bars spaced about a quarter inch apart. It is designed to screen out rags, wood, plastics and other floating objects that could clog pipes or disable pumps if not removed. These solids are washed, compacted and then sent to a landfill.
Aerated Grit Chamber
The grit chambers are designed to slow the velocity of the wastewater down so that grit, sand, and other abrasive materials can settle to the bottom. The grit is then removed and washed before disposal at a landfill.
In-Line Equalization Basin
The in-line basin has a volume of 2.5 million gallons. It provides equalization of diurnal flow variation to achieve a constant or nearly constant flow rate and wasteload through the downstream treatment processes.
Off-Line Stormwater Basins
There are three off-line storage basins with a combined capacity of 4 million gallons. They are used to temporarily store excess wet weather flow. This wastewater is then recirculated back to the in-line basin when space is available.
Activated Sludge Process
The activated sludge process is an aerobic biological process. The goal of this process is to remove the carbonaceous waste contained in wastewater by providing an environment for bacteria and other microorganisms to grow and consume the organic waste. It consists of two aeration tanks and two clarifiers. In the aeration tanks, the return sludge
flow is combined with the wastewater. Diffused air is added to provide oxygen so that organic pollutants can be consumed by the microorganisms. This biomass then separates in the clarifiers. The pumped underflow from the clarifiers is either directed as a return sludge flow to the aeration tanks to mix with the incoming wastewater, wasted to the sludge thickeners for ultimate disposal, or sent through the phostrip process for phosphorous removal (see phostrip). The clear effluent travels to the next process for further treatment.
Phosphorus removal is one of the mandates at our plant.. The Phostrip system accomplishes this by working in conjunction with the activated sludge system. By control of the aerobic sludge pumps a small portion of the settled sludge from the activated sludge clarifiers, is diverted to an anoxic (without oxygen) phosphorus strip tank where it will be retained for a time sufficient to induce phosphorus release. A portion of the activated sludge clarifiers effluent and the elutriation flow from the reactor clarifer are sent through the stripper from the bottom resulting in an elutriation overflow high in phosphorus.
At this point the stripper overflow with its high concentration of phosphorus is treated with lime to increase the pH to greater than 9.0 and then flows to the reactor clarifier where the phosphorus, and any other solids, are settled out and sent to the gravity thickeners for disposal. The phosphorus stripped sludge is returned to the activated sludge tanks to uptake more phosphorous from the incoming wastewater.
The nitrification system consists of two plastic media trickling filters and two clarifiers. The clear effluent from the activated sludge clarifiers is pumped to the top of the 21.5 foot trickling filters and distributed over the plastic media. The nitrogen in the wastewater is oxidized to nitrite and nitrate by a population of microorganisms attached to the plastic filter media. The slime layer increases in thickness and eventually washes off (termed sloughing). These solids that are sloughed off settle in the nitrification clarifiers and are wasted to the gravity thickeners. The clear effluent flows to the denitrification process from July through October and to the chlorination system from November to June. We are only required to denitrify from July to October.
The denitrification system is a biological process by which nitrates are removed from the wastewater and converted to nitrogen gas. The first stage is an anoxic mixing tank where the dissolved oxygen is kept below 2.0 ppm and a carbon source is added to the wastewater. Diluted corn syrup and/or a manufactured glycerin product is used at the Lansdale plant as a carbon source.
The second stage is an aerated stabilization tank where the microorganisms oxidize any remaining corn syrup. Since any excess syrup bleeding through this process would result in excess B.O.D. in the effluent, it is important that the feed rate is monitored closely.
The third and final stage is the denitrication clarifiers, where the sludge is settled out in these tanks, and either returned to the first stage or wasted out of the system entirely. The entire process is a seasonal one, and is only used from the first of July to the end of October.
Wastewater contains disease causing bacteria, viruses, and parasites. These microorganisms are commonly referred to as pathogenic organisms. Many of these pathogenic organisms are removed in the preceding plant processes. Chlorine is added to the treated wastewater to kill essentially all of the remaining pathogenic organisms before being discharged to the receiving stream. This process, termed disinfection, prevents the spread of waterborn disease. Chlorine solution is added at the head of the chlorine contact tank, which is a maze configuration to provide mixing and contact time. Adequate chlorine is added to ensure disinfection of the treated wastewater. This is controlled by the amount of residual chlorine remaining at the end of the contact tank.
Dechlorination is the last process control that takes place at the wastewater treatment plant. Before the treated water is discharged into a tributary of the Neshaminy Creek, Sodium Bisulfite is added to remove any chlorine residual. The Sodium Bisulfite reacts very quickly with the chlorine residual to form non-toxic chloride, thus protecting the Neshaminy Creek from chlorine toxicity.
The gravity thickeners at Lansdale are operated to stabilize and concentrate lime treated sludge prior to hauling liquid sludge off site to a local incinerator for disposal. All waste sludge from plant processes is mixed with lime and pumped to the gravity thickeners for concentration. The concentrated sludge is very gently agitated by moving rakes and vertical pickets which dislodge gas bubbles and keeps the sludge moving toward the center well from which it is pumped. A sludge blanket is maintained on the bottom of the thickener to aid in concentrating the sludge.