Operation of the North Wastewater Treatment System
 

The Wastewater Treatment Plant was constructed as a primary treatment facility in 1954 and upgraded to secondary treatment in 1975. The plant was renovated and expanded in 1991 & 1998 (additional aeration basins, secondary clarifier and RAS pumping station). The primary settling clarifiers were taken off line in 2001.

The current plant is operated 24 hours-per-day, 365 days-per-year and provides full primary and secondary treatment. The plant has the capacity to provide treatment to remove over 98% of the contaminants from the water at an average flow of 7.5 million gallons per day (MGD) with a design flow of 15.0 MGD. This process is a dynamic process which must be properly monitored and controlled to assure maximum effectiveness with changing flow rates and organic loadings.

Wastewater is the potable water supply of a community after it has been used in homes, businesses, and industries and has been contaminated by various uses. Wastewater Treatment utilizes a combination of processes which alter and reduce the contaminants and removes them from the water. The purpose of a wastewater treatment plant is to accelerate the natural processes by which water is purified.

As wastewater enters the plant, it passes through a row of two closely spaced coarse, vertical bar screens. Large objects which are trapped by these screens are removed by automatic mechanical rakes. The wastewater then flows to a centrifugal-type grit removal tank in which sand, cinders, and gravel are removed from the wastewater. The flow then passes through two fine bar screen removal units. After the initial screening and grit removal, the wastewater still contains high concentrations of dissolved, small suspended solids and organic materials. This process is known as preliminary treatment.

After the wastewater leaves preliminary treatment it enters a secondary treatment system. The secondary process uses an extended aeration variation of the activated sludge process. The water is first pumped to a very large aeration tank where it is mixed with air and activated sludge. The detention time in this basin is 18 – 60 hours depending upon influent flow. During this time the aerobic bacteria and microorganisms present in the activated sludge consume by “eating” most of the organic matter. These bacteria and microorganisms, referred to in the industry as “bugs”, depend on oxygen for their respiratory systems the same as humans. For this reason large quantities of air are pumped into the aeration basin where air bubbles rise continually up through the liquid to provide sufficient oxygen for the bugs to breathe.

Four (4) groups of bugs do most of the “eating” in the activated sludge process. The first group is bacteria which eat the dissolved organic compounds. The second and third groups of bugs are microorganisms known as free-swimming and stalked ciliates. These larger bugs eat the bacteria and are heavy enough to settle by gravity. The fourth group are microorganisms known as Suctoria & Rotifers which feed on the larger bugs and assist with settling.

An interesting item about the bacteria that eat the dissolved organics is that they have no mouth. The bacteria have a fascinating characteristic in that their “fat reserve” is stored on the outside of their body. This fat layer is sticky and is what the organics adhere to.

Once the bacteria have “contacted” their food, they start the digestion process. A chemical enzyme is sent out through the cell wall to break up the organic compounds. This enzyme, known as hydrolytic enzyme, breaks the organic molecules into small units which are able to pass through the cell wall of the bacteria.

In wastewater treatment, this process of using bacteria-eating-bugs in the presence of oxygen to reduce the organics in water is called activated sludge. The first step in the process, the contact of the bacteria with the organic compounds, takes about 20 minutes. The second step is the breaking up, ingestion and digestion processes, which takes four (4) to 24 hours.

The fat storage characteristic of the bacteria is also an asset in settling. As the bugs “bump” into each other, the fat on each of them sticks together and causes flocculation of the non-organic solids and biomass.

From the aeration tank, the wastewater, now called mixed liquor, flows to secondary clarification basins to allow the flocculated biomass of solids to settle out of the water. The solids biomass, which is the activated sludge and contains trillions of bacteria and other microorganisms, is used again by returning it to the influent of the aeration basin splitter box for mixing with the raw sewage and ample amounts of air.

Henderson’s Wastewater Treatment Plant is unique in that a third or tertiary clarification section consisting of two (2) tertiary clarifiers are used to remove a higher degree of suspended solids prior to discharge to the Ohio River. These two tertiary clarifiers also act as the chlorine contact basins.

The final phase of treatment consists of the addition of chlorine to the effluent water from the secondary clarification basin. Chlorination accomplishes disinfection by killing more than 99% of the remaining bacteria. The disinfected water is then de-chlorinated, using Sodium Hydrogen Sulfite (aka Bisulfite) to remove the toxicity effect of chlorine on certain forms of aquatic life. The water is finally discharged through a submerged outfall pipe into the Ohio River.

As a result of the biochemical decomposition, the volume of the sludge is reduced and the organic solids, such as sugar, carbohydrates and fats, are broken down into smaller inert organic and inorganic solids. Carbon dioxide, methane and water are the by-products of organic reduction process.

When digestion has been completed, the sludge is withdrawn from the digesters to dewatering devices known as belt filter presses. The digested sludge consists of approximately 97 - 98% water, therefore it is necessary to remove enough of the water to produce a material which is dry enough to be hauled in trucks. Water is removed from the sludge by use of porous fabric belts which pass around a series of rolls at increasingly higher pressure. For more effective separation of the water and solids, the sludge is first treated with a liquid cationic polymer which chemically causes the water and solids to separate. The polymer treated sludge is introduced as slurry between the fabric belts. As the belts wrap around the rolls, a large quantity of water is squeezed out of the sludge and escapes through the porous belts. The process of removing the water is somewhat similar to that used by wringer clothes washing of years past. The dewatered sludge cake at 14 - 18% solids is scraped from the belts on the discharge end of the presses where it drops onto a belt conveyor and is deposited in a storage building. The sludge cake is pushed into a hopper at floor level onto a belt conveyor and loaded into trucks. The sludge cake is taken to a local farm for land application and/or composting.

Supporting all the various phases of the wastewater treatment and sludge disposal process is the laboratory where numerous tests and analyses are performed on a daily basis to assure that the process variables and life support conditions for the bacteria and microorganisms are within optimum ranges. The laboratory also helps meet our required reporting for State & Federal reports.

Although not an integral part of the actual treatment process, but critical to the success of the process, is the Industrial Pretreatment Program. This program is an EPA permit requirement established by the Clean Water Act of 1977, the purpose of which is to guard the quality of the wastewater entering the plant. The program consists of monitoring, surveillance and regulation of the wastewater discharge from major industrial contributors to the municipal wastewater collection (sewers) system. Discharge of certain types of chemicals and metal compounds can have a severe toxic effect on the wastewater treatment process and therefore must be regulated to prevent such discharges. The characteristics of the discharges from some industries are such to require that the industry construct and operate a contaminant loads prior to discharge to the municipal wastewater collection system.

The operation of the Henderson Wastewater Treatment System contributes to improvement of water quality in the Ohio River, and as a result, helps to provide recreation and health benefits for the entire community. The Ohio River is, in certain respects, one of the community’s most valuable assets and the protection of this asset is a primary goal in the operation of the Wastewater Treatment System. The higher water quality also helps, proportionally, to provide a cleaner water source for many cities downstream of Henderson all the way to the Gulf of Mexico.