Wastewater Treatment

The City of Lawrence Wastewater Treatment Plant, located at 1400 East 8th Street, has the capacity to treat approximately 12.5 million gallons per day, with peak flows of 25 million gallons per day through the normal process and an additional 40 million gallons through the Excess Flow System during extreme rain events. City staff are responsible for the plant's efficient operation and maintenance.

Wastewater is defined as water that has been used for domestic or industrial purposes. Wastewater treatment is the physical, chemical, and biological processes used to remove pollutants from wastewater before discharging it into a water body. The various treatment stages are described below.

Influent Pump Station:

The influent pump station receives wastewater from the wastewater collections system through sewer lines located 25-30 feet underground. At the front of the pumping station wastewater passes through mechanical bar screens that remove large debris (3/4" or greater) such as boards, rags, and paper, which would clog pumps and damage equipment. These materials are mechanically raked off and landfilled.

The influent pump station houses 4 pumps with a pumping capacity of approximately 25 million gallons per day. On an average day they will pump approximately 9 million gallons of wastewater, lifting it 25 feet and delivering it to the grit chamber so that it may continue through most of the plant processes by gravity. Although the collection system is designed to receive only sanitary wastewater, imperfections in the system allow groundwater and rainwater to infiltrate and inflow (I&I) into the system. Peak flows during rain events can exceed the 25 MGD pumping capacity of the influent pump station, which then requires the excess flow to be treated through the excess flow system.

Grit Removal:

The screened wastewater proceeds to the grit complex where heavy inorganic solids, such as sand or gravel (grit) are allowed to settle while lighter organic solids are kept in suspension. The grit is removed, washed and dewatered by mechanical methods prior to disposal in a sanitary landfill.

Primary Sedimentation:

The wastewater is divided between two basins, which use gravity to settle the solids to the bottom of the basin where they are removed for treatment. This process uses detention time of approximately 3-4 hours to allow for the particulates in the wastewater to settle out. Approximately 65% of the suspended solids are removed in his process with the remainder being dissolved and un-settlable pollutant solids.

Secondary Treatment of Activated Sludge:

Aeration basins are used to accelerate the naturally occurring breakdown of waste that occurs in the environment. The alkalinity, dissolved oxygen, and mixing are adjusted to encourage optimum microbial activity. Diffused air ensures that the contents of the basin are mixed and to provide oxygen. The dissolved oxygen is used by the microbes to respire (breath) as they consume food (wastewater solids). As the microbes consume the food their population increases. The quantity of microbes is adjusted to balance with the volume of waste received.

Clarification:

Final sedimentation basins are used to hold the wastewater for a period of time. Through the process called flocculation the microbes and remaining waste material form particulate that are heavier than water and settled to the bottom of the basins. Portions of the solids settled are continually removed and sent back to the aeration basins to maintain the microbial population there. The excess solids are periodically removed and sent to the anaerobic digesters.

Disinfection:

The wastewater (or called effluent at this point) proceeds to disinfection where liquid chlorine is added to kill most of the remaining pathogens (disease causing organisms).

Due to the potential toxic effects of chlorine on aquatic life, sodium bisulfite is then added to remove any residual chlorine prior to discharge into the Kansas River.

Excess Flow System:

Due to imperfections in the collections system, flow to the wastewater treatment plant can increase significantly during substantial rain events. Due to the need to treat this excess water (mostly dilute), a system (called Actiflo) was developed to be used during these periods of high flow. The excess flows to the WWTP greater than 25 million gallons per day are pumped from the head of the plant by-passing normal treatment and through fine screens (to screen out any objects over the size of a pencil eraser). After screening, the flow proceeds to the Actiflo basins. There, ferric chloride, polymer, and micro-sand are mixed with the wastewater to form ballasted floc, which settles quickly and is removed from the flow. The clear effluent from Actiflo flows to a chlorine contact basin that is dedicated to this flow, where liquid chlorine is added for disinfection and sodium bisulfite is added to remove the residual chlorine (as takes place under normal treatment). The effluent from this process is combined with the normal plant effluent and discharged to the Kansas River. Small rain events may be diverted through the fine screens to the excess flow storage basin until flow subsides and it can be returned to the head of the plant. Although this system has been widely used in drinking water treatment, this was the first system to be installed and operational in the United States for use in wastewater treatment.

Digestion:

The solids that are removed from the primary and secondary treatment processes are stabilized by anaerobic digestion. This process is maintained at 98oF. In a controlled environment, a complex population of bacteria work in the absence of oxygen (called anaerobic respiration) to reduce and stabilize biodegradable waste products prior to removal. End products of this process include a stabilized organic material, carbon dioxide and methane. The boilers used to maintain the digester temperature use the methane from this process as their fuel source. The stabilization process takes from 14-30 days. The digested solids (now called biosolids) are removed from digester for dewatering.

Biosolids Dewatering:

Two 2-meter belt filter presses are used to remove excess water from the digested solids. The water is removed using gravity to drain excess water away and applied pressure between two perforated belts that squeeze as they move over rollers. The biosolids enter the presses at approximately 2% solids and leave the presses at approximately 20% solids. The biosolids are transported to storage bays via a series of conveyors. The excess water removed from the biosolids (called filtrate) is returned to the headworks of the plant for treatment through the normal process.

Biosolids Storage:

Storage exists for approximately 4000 cubic yards of biosolids. These bays are designed for 120 days of storage at the year 2020. It is divided into two cells to allow for removal from one while the other is being used. Each cell has large water tight gates which when opened allow for access with a front end loader for easier removal of the material.

Biosolids End Use:

Approximately 95% of the biosolids produced at the Lawrence Wastewater Treatment Plant are beneficially reused as a nutrient source and soil conditioner on privately owned agricultural land. The treated biosolids contain three key crop nutrients - nitrogen, phosphorus, and potassium, as well as other nutrients in trace amounts. Organic matter supplied by the biosolids improves soil structure, increases the soil's ability to absorb and store water, and increases availability of the nutrients to the crop.

The City of Lawrence has recycled biosolids in this manner since the mid-1970's on City owned and privately owned agricultural fields. With the adoption of the federal regulations (EPA 40 CFR Part 503) which regulate the disposal and reuse practices for biosolids, the City developed its Biosolids Beneficial Reuse Program. Through this program, biosolids are applied to local agricultural land at no cost to the participating farmer or landowner. A private contractor that specializes in the land application of biosolids takes soil samples, compiles reports and performs the application activities with oversight of city staff. The program applies biosolids and its nutrients to approximately 400 acres annually.

The remaining 5% of biosolids production is set aside for residential use by the public. The material is tested extensively and upon meeting compliance with federal regulations is made available for use on residential lawns, gardens, flowers, and other residential uses. The practice of making biosolids available to the public in Lawrence for residential uses has taken place since 1956 when the plant was first constructed.

The biosolids must meet stringent testing and procedural requirements. The City's reuse activities are in complete compliance with all Environmental Protection Agency regulations and Kansas Department of Health and Environment guidelines. The City of Lawrence is committed to the safe, economical, and environmentally friendly reuse of this valuable material.

To learn more about wastewater, biosolids, or the City's EMS for Biosolids, please contact the Utilities Department at 785-832-7800.



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