Evaluation of dewatering alternatives &amp; preliminary design for the Evansville Water Treatment Plant
Creators: name: Miles, Lauren
name: Williams, Shelby
Publisher: name: Unknown
Year: 2014

description: The Evansville Water Treatment Plant supplies water to over 60,000 customers in Vanderburgh, Gibson, and Warrick counties. On average, the plant treats 23 million gallons of water per day. The plant is located on the Ohio River near downtown Evansville, IN. Water is treated by conventional processes, including coagulation, flocculation, sedimentation, multi­media filtration, and disinfection. Sedimentation takes place in large reactors known as "clarifiers". The multi-media filters are regularly backwashed to remove particulate matter that has accumulated in the media. 
The Evansville Water Treatment Plant's current National Pollutant Discharge Elimination System (NPDES) permits allow solid waste from treatment to be dumped directly into the Ohio River. Currently, settled solids removed from two pairs of primary clarifiers and solids generated from filter backwashing operations are discharged directly into the Ohio River without treatment. It is estimated that the waste streams from these processes discharge in excess of20,000 pounds of solid material per day. It is expected that when these permits expire, the plant will be required to implement a mechanical dewatering system to dispose of its solid waste. The goal is to separate the solid material from the water. The solids, once concentrated, are referred to as a "sludge cake" and can be disposed of in a landfill or used for land application. The main objective of this project was to evaluate dewatering options for the plant and provide a preliminary design of the main components. The main components for the system include a flow equalization basin, two gravity thickeners, and a mechanical dewatering facility. 
The treatment plant has three waste streams that contribute to its solid waste. They are coagulation sludge from the north and south primary clarifiers, and solids from filter backwash. Laboratory tests were conducted in order to determine the characteristics and solids concentrations of each of the waste streams. It was found that the percent total solids content of each of these streams was very low, (&lt;1%). The three waste streams also had variable suspended solids concentrations. These tests led to the consideration of an equalization basin and gravity thickeners to increase the solids content of the three streams. 
With flow rate information provided from the plant, an equalization basin design was created. An equalization basin with mechanical mixing is necessary to mix all three waste streams which all have different flow rates and different solids concentrations leaving the plant. Sludge form the north and south plants continuously discharge at the same rate, while solid waste from the filters has a variable discharge during filter backwash cycles. The 1.55 million gallon equalization basin, with two equally sized compartments was designed to stabilize the highly variable waste streams and provide a more consistent composition to downstream processes. 
Thickening of the sludge is necessary prior to mechanical dewatering because the percent solids were very low. It was determined that the method of thickening would be gravity thickening. Gravity thickening allows the solids to settle out while decanting the water off of the top. Two 45 feet diameter circular gravity thickeners were designed and are expected to increase the solids content to approximately 6% prior to mechanical dewatering. It is also expected that as the plant continues to grow, other gravity thickeners will need to be added to the system. 
Four mechanical dewatering alternatives were evaluated. They include a belt filter press, gravity belt thickener, a centrifuge, and a plate and frame filter press. These four alternatives were compared on the basis of performance, serviceability, and cost, an economic analysis was done on each of the machines. 
The recommended mechanical dewatering equipment based upon performance, 
serviceability, and cost analysis was the gravity belt thickener. The gravity belt thickener is the most cost effective of the four alternatives, and also handles lower percent solids compared to others. One gravity belt thickener operating 12 hours per day can handle the high loading rate of 301 gpm. Due to automated operating systems, it requires less operation attention. Two gravity belt thickeners are needed but only one will be operational, the other will be used for backup. The total operation and maintenance costs are estimated at $68,266 per year. Polymer usage is based on a 0.5% stock solution with a concentration of 100 mg/L. The estimated use of polymer with this concentration is 65,700 lbs/year and the cost is $1.30 per pound. The yearly cost of Polymer usage is estimated to be $85,810. Ultimate disposal of the Alum sludge should be landfilled. The cost to dispose the estimated 10 tons/day of solids at $25/ton in the Evansville landfill is $91,250 per year. The total annual cost for this design will be $244,926 per year. The total capital cost of this project will consist of a $1,940,000 equalization basin, two $125,000 gravity thickeners, and two $175,000 gravity belt thickeners. The total capital cost for the proposed design will be $2,540,000.