Treatment Process

The wastewater from the city and a portion of Logan Township is conveyed to the treatment facilities also known as the Westerly and the Easterly, via three outfall sewers. The Westerly Facility also receives wastewater from a portion of Allegheny Township by a separate outfall. The annual average flow of the Westerly Facility is 10.8 MGD with a permitted design hydraulic capacity of 16.2 MGD. The annual average flow of the Easterly Facility is 9.0 MGD with a permitted design hydraulic capacity of 11.25 MGD. Each facility is capable of handling flows of up to 20 MGD during wet weather. These treatment facilities provide advanced secondary treatment capability normally removing 95% of organic and suspended solids.

Both treatment facilities are controlled and monitored by identical computerized process control systems. Each facility is made up of five unit processes; preliminary treatment, flow equalization, activated sludge process, clarification & disinfection and, biosolids handling process. Plant operators, through the computerized control system screen, can monitor the entire treatment process, and they can also manually control equipment from this centralized system. Flows, tank levels, valve positions, pH and dissolved oxygen levels, and the operation of blowers, pumps, flow gates and equalization tanks are all controlled automatically from data fed to the computer.

As the wastewater enters the preliminary treatment unit, the flow is diverted into channels which are equipped with mechanically cleaned bar screens. These screens provide the removal of larger debris and trash entering the facility. Immediately downstream from the bar screens the wastewater enters the aerated grit removal unit (s) where smaller debris (sand, gravel, cinders, etc.) are removed. Removal of grit and other debris is extremely important for the protection of pumps and equipment. The grit and debris are transported to a permitted landfill for proper disposal.

To minimize the impact of peak wet weather flows, automatic controlled equalization tanks are provided. Flows in excess of 20 MGD are diverted into the equalization tanks and stored until the flow entering the facility recedes. The stored wastewater is then pumped into the system for proper treatment.

The next stage of treatment is the Activated Sludge Process. This process is utilized to convert non-settleable substances in finely divided, colloidal and, dissolved form, into settleable sludge and to remove the sludge, thereby providing a high degree of treatment. The activated sludge process depends on groups of microorganism, primarily bacteria and protozoa, living on wastewater solids. as a purifying medium. These organisms are maintained in an aerobic environment by introducing air into a mixture of activated sludge and wastewater, after which the activated sludge is separated from the treated wastewater by settling. It is generally accepted that in the activated sludge process "adsorption" plays an important role in the removal of organic materials from the wastewater.

In the aeration tanks the incoming wastewater and the activated sludge are mixed together by introduction of air. The aeration provides ample supply of oxygen for the growth and development of aerobic bacteria in the wastewater and activated sludge and promotes oxidation of the organic matter in the wastewater. The fine colloidal and suspended solids form a "floc". The principal action in the formation of the "floc" is generally assumed to be adsorption. The colloidal solids are adsorbed by the floc already formed and are thus removed from the wastewater.

The oxidation of organic matter in the wastewater and the formation of floc are much more rapid if the activated sludge that has been previously produced in the process of treatment is added to the incoming wastewater at the influent end of the aeration tanks(s). A plausible reason for this speeding up of the clarification process is that the activated sludge contains large numbers of aerobic bacteria that renders the sludge an active agent in oxidizing the wastewater. The continuous motion of the wastewater in the aeration tank is essential in order to bring all of the solids of the new wastewater in contact with the previously produced activated sludge and to prevent any settling of the sludge particles.

The aeration effluent then flows into the circular clarification tank(s) where the separation of sludge and the purified liquor is accomplished. When more than one clarifier is in use it is important to maintain the flow to the tanks at a uniform rate. The velocity of the incoming wastewater is reduced to a fraction of an inch per sec. to allow the solid material ("floc") to settle to the bottom as the liquid continues over a weir into the effluent channel. A baffle is placed in front of the effluent weir to prevent floating material (scum) from reaching the weir. A rotating scraper arm projects above the water surface to push the scum toward a trough where it is skimmed off and pumped to the primary sludge digestion tank. Proper baffling for good distribution and good skimming practices are most important as any solids or scum carried over the final weir go directly into the receiving stream.

The solid material or sludge floc is deposited onto the floor of the tank. This sludge is drawn off be means of a rotating scraper and slip tubes by hydrostatic pressure. It is important that the sludge be kept flowing from these tubes at all times. It is also essential. that the sludge be drawn from each clarifier at a uniform rate. The sludge removal from the clarifier is continuously pumped to the head of the aeration tank(s) and mixed with the incoming wastewater. The concentration of activated sludge (mixed liquor) is maintained at a predetermined level by the facility operator.

The clarified liquid flows into the effluent channel and into the ultraviolet disinfection reactor{s). As the flow passes through the reactor(s) the ultraviolet light disrupts DNA in bacteria, viruses, molds, algae and other microorganisms causing cell death or inactivity. The disinfected water is then discharged into the receiving stream. The Westerly Facility empties into the F rankstown Branch of the Juniata River and the Easterly Facility empties into the Little Juniata.

As the concentration of solids increases in the aeration system, a portion of the retum activated sludge from the clarifier is wasted from the system and pumped to a gravity belt thickener. A polymer is added to enhance the separation of water and solid. A sludge of between 3 and 4 percent is pumped from the thickener to a primary digestion tank. As the amount of food decreases the microorganisms become competitive and eventually begin to rely on each other for food. The sludge is transferred to a secondary digestion tank as necessary to provide enough time for the aerobic stabilization of organic matter to a relatively stable or inert organic and inorganic compound or biosolid. (Usually 20 to 40 days hydraulic retention time).

The digested or stabilized biosolids are drawn from the bottom of the secondary digester and pumped to a belt filter press. A polymer is again added to enhance the separation of liquid and solid, resulting in a sludge cake of 18 to 20 percent solid. The biosolids are stored on site for land application on approved and permitted farms or hauled to a permitted landfill for disposal.