Solving a Water Plant Sludge Problem 

Introduction:
For over 10-years the Lincoln County Lake Norman Water Treatment Plant stored the plant’s backwash water and settling basin solids in a concrete tank originally built to store finished water.  The water plant utilized a one million gallon prestressed concrete tank that was built for finished water storage to hold the water plant solids.  An accumulation of sludge in the tank over the years created a burdensome problem that was solved in 2001 with a solids handling project that allowed the prestressed concrete tank to be converted back to it’s intended use as a clearwell.

 This 3-MGD water plant was a state-of-the-art facility when it was built in 1990 except for handling backwash waters and solids accumulating in the settling basins.  The County fun, due to ading shortfall, converted the one million gallon clearwell to a storage tank for backwash water as a short-term fix to the funding problem.  Backwash water was recycled back through the water plant for treatment when the water plant was first built.  Later a NPDES discharge permit was obtained to pump and discharge the supernatant directly back into Lake Norman while continuing to store the solids in the on site tank.  The short-term fix turned into a major operational headache before the sludge-handling problem was solved.

 A  solids handling project, completed in 2001, included construction of two concrete tanks for storage and gravity thickening of waters wasted from backwashing and solid removal in the settling beds.  These settled solids are now thickened using a vacuum assisted sludge drying bed while the supernatant from the gravity thickeners is returned to Lake Norman through a gravity drain.  The County evaluated using belt filter press, centrifuge, vacuum beds and gravity beds before deciding to construct the vacuum drying beds. The vacuum bed option proved to have the lowest capital cost, lowest annual operation and maintenance cost and the lowest twenty-year present worth cost of the various options considered.

Background:
Lincoln County was formed in 1779 from Tryon County.  It is in the southwestern section of the State and is bounded by Mecklenburg, Gaston, Cleveland, Catawba and Iredell counties.  The County draws water from Lake Norman, a manmade reservoir on the Catawba River located on the eastern boundary of the County.  The lake water is treated at the County water treatment facility, located adjacent to Lake Norman, and the treated water is distributed throughout most of the County to over 7,000 customers.  Raw water turbidities range from 3 to 24 NTU with an average turbidity of 8 NTU.  The raw water contains very little natural solids.  Solids removed in the treatment process are mostly chemical solids from chemicals used to treat the water.  Collected water samples showed poor settling characteristics under lab conditions with a dry suspended solids content of 0.43%.  A sludge sample only thickened to a dry solids concentration of 0.86% after settling 24-hours.

The water plant is currently configured to pump raw water from the lake through a series of flocculator tanks, rectangular shaped gravity settling basins and dual filter tanks.  Solids that are settled out in the settling basins are removed from the floor of the basins with a vacuum sludge removal mechanism that sweeps the floor at preset intervals set by a timer controller.  Typically the basins are swept once a day for a period of approximately 20-minutes each.  About 30,000 gallons of water, or 1% of the water treated daily, is wasted with each cleaning of solids from the basins.  The second source of solids generated by the water treatment process is from backwashing the gravity filters.  Currently there are two filter beds.  Each filter requires about 50,000 gallons of process water to backwash, 1.7% of the treatment plant capacity.  Prior to 2001, when the vacuum beds and gravity thickeners were placed into service, all backwash water and settling basins solids was gravity drained and stored in a 1.0-MG prestressed concrete tank originally constructed to store finished water.  Over the first 10 plus years of operation the solids were stored in the tank.  The water plant operators would periodically draw supernatant off the tank and return the water initially through the water plant and later to Lake Norman with a NPDES discharge permit.  The solids remained stored in the tank over this period of time till the tank was in effect full and something else had to be done.

 Solids Handling Design:
In March of 2000 the County ask WK Dickson to help find a solution to the water plant sludge problem.  Working with the water plant ORC, Mr. Larry Warren, and the County Engineer, WK Dickson evaluated various alternatives to handling solids including belt filter press, centrifuge, vacuum drying beds and gravity drying beds.  Sludge dewatering proposals were received from major equipment manufacturers and site visits were made to various water plants using a variety of sludge dewatering methods.  After evaluating all alternatives both the belt press and vacuum beds were found to be viable alternatives for dewatering the sludge.  Centrifuges and gravity beds was eliminated as viable alternatives early in the evaluation as a result of equipment costs, performance data and discussions with other users.  Both the belt press and the vacuum bed manufacturers showed through lab pilot testing to be able to dewater the Lincoln County sludge to a 15% dry solids, producing a dry cake that will pass a paint filter test.  Present work cost analysis showed the belt filter press and vacuum bed cost as follows:

Bids were received in December 1999 and Ray Smith Construction Company of Newland, NC was the low bidder.  Construction of the facilities was completed in June 2001 for a total cost of $1,110,269.

 Sludge Facilities:
The sludge handling facilities includes; two gravity thickener tanks, one vacuum drying bed and a building housing; controls, sludge pump, vacuum pump and a polymer feed system.  The backwash water and sludge vacuumed from the settling basins flows by gravity to the thickener tanks through a 20-inch pipe.  Piping and valves are provided to enable the two tanks to work together or as separate tanks to store the sludge until dewatered.  The settled sludge is drawn from the bottom of the thickeners by a centrifugal pump and spread over the vacuum drying beds using a total of seven 2-inch discharge pipes with valves to distribute the sludge over the bed.  Supernatant is drawn from the thickener tanks and discharged to Lake Norman through a 12-inch gravity drain.  A vacuum pump and polymer system is available to dewater the sludge on the beds.  The sludge filtrate is returned to the gravity thickeners through an on site pumping station and force main.  The dried sludge is removed from the beds with a small bobcat and trucked to the County’s solid waste site for disposal.

Gravity Thickener Tanks: Each tank is 35-feet in diameter with a 22’-3” vertical side water depth and a 10’-10” conical bottom..  Each tank can store 199,000 gallons.  Two 6-inch perforated pipes ring the diameter of the tanks above the conical bottom at two different levels to remove the supernatant after the sludge has settled.  The supernatant is drained through a 12-inch pipe and discharged directly into Lake Norman.  Piping and valves are available to control flow to the two tanks and to transfer sludge between the two tanks.  A 8-inch suction pipe is provided to draw sludge from the bottom of each thickener with a 700 gpm sludge pump.  

Vacuum Drying Bed: a 1,240 square foot vacuum bed sized for 6.0-MGD of treatment capacity is provided.  The bed is 16-feet wide by 76’-8” long with seven 2” feed pipes equally spaced along one side of the bed.  An 8-inch filtrate drain runs the length of the bed and is gravity drained to an on site submersible pumping station.  The filtrate is drawn off the tank and pumped back to the gravity thickeners.  Polymer is blended with the sludge when the vacuum bed is loaded from the thickener.  A 2-hp vacuum pump is tied to the filtrate drain under the bed and operates to draw a vacuum on the bed after the operator loads the bed from the thickener tank.  U.S. Environmental Products, Inc. furnished the vacuum drying bed and associated equipment.

 Sludge Pump: The County uses a Godwin self priming pump to draw sludge from the thickener and pump the settled sludge to the vacuum bed.  Polymer is blended with the sludge at the discharge of the sludge pump.  The operator has a control station adjacent to the vacuum bed to control the pump and the polymer feed.  The sludge pump, polymer tank and vacuum pump are housed in a building located adjacent to the vacuum bed.

 Sludge Dewatering:
The operators store sludge in the gravity thickener tanks until they are ready dewater the sludge.  The sludge settled out in the conical bottom of the thickeners and the supernatant is drained off and discharged back to Lake Norman.  Currently, the operators control the supernatant removal manually.  There are also timer controlled valves provided to allow supernatant drains to open and close with a 24-hour time clock.

The process of drawing sludge from the tanks and pumping onto the drying bed takes approximately 30-minutes.  The operator has already prepared the polymer in the 700-gallon mixing tank prior to operating the sludge pump.  The polymer is blended with the sludge as the sludge is loaded on the vacuum bed.  The operator can control the speed of the sludge pump and the rate of polymer dosage from a control station located adjacent to the vacuum bed.  There are also control valves on the sever (7) bed inlet pipes to control the loading of the bed.  After the process of loading the bed is completed, the vacuum bed is allow to sit for the next 24-hours at which time filtrate drains through the bed to the filter drain located under the bottom of the bed.  The next day the operators will operate the vacuum pump and complete the drying process.  This generally takes about 30-minutes and leaves a dry cake of about 10% to 12% on the beds.  Pilot testing during the design showed a dried sludge cake of about 15% could be achieved with the addition of a small amount of builders sand added in from of loading the bed with sludge.  This involved placing a bucket of sand in from of each discharge pipe prior to pumping the sludge onto the bed.   The County thus far has not seen the need to use the sand to enhance the dewatering.

 After the vacuum has been pulled on the bed, the operator uses a small bobcat to remove the sludge.  The sludge is removed from the bed and placed on an asphalt area adjacent to the bed where the sludge is left for a period of time to air dry.  It generally takes 20 to 30 minutes to remove the sludge from the bed.   The air drying of the sludge before trucking the sludge to the land fill has proven to be very effective and results in a sludge cake that is easy to handle.

 Conclusion:
The vacuum bed has proven to be an effective method to dewater sludge at the County’s Lake Norman water plant.  The sludge handling project has allowed a much needed clearwell tank to be converted from a sludge storage tank to its intended use.  Key to a successful system is having effective gravity thickener tanks to store sludge until the operators are ready to dewater the sludge.  Also key are sludge pumps and controls to allow the operators to easily remove the sludge from the thickeners and transfer the sludge to the vacuum bed for dewatering.

 Author(s):       Michael L. Wolfe
                        WK Dickson
                        616 Colonnade Drive
                        Charlotte, NC 28205
                        704-334-5348
                        704-334-0078 (Fax)