Technical Memorandum
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Technical Memorandum To: Chris Ehlers – City of Brentwood Casey Wichert – City of Brentwood From: Michael Ducker, P.E. Chad Coleman, P.E. Reviewed By: Greg Chung, P.E. – West Yost Associates David Anderson, P.E. – West Yost Associates Date: March 25, 2015 Revised April 15, 2015 Project: Brentwood Wastewater Treatment Plant Subject: Tertiary Filters PROBLEM STATEMENT The City of Brentwood currently experiences high electricity costs due to peak loads from the air scour blowers for the deep bed sand filters. The City is interested in exploring the use of cloth media filters instead of deep bed sand filters since cloth media filters require no air scour blowers. This replacement could be a means of reducing the peak electrical load and associated costs of tertiary filtration. The purpose of this report is to summarize the issues and quantify the payback time required to recover the capital investment assuming the filters are changed. If the use of a cloth filter is determined to be feasible, then cloth filters may be installed for the Phase 2 expansion in conjunction with the existing Phase 1 deep bed sand filters. If a Phase 2 expansion of cloth filters proves to reduce the power consumption and load requirements, then the existing Phase 1 sand filters may eventually be replaced. This memo will include a comparison of the qualitative and quantitative costs and benefits associated with the installation of a cloth media filter. DISCUSSION OF EXISTING TREATMENT PROCESS The City of Brentwood WWTP is a suspended growth activated sludge process. The plant operations and processes included the following: • fine screens • grit classifiers • denitrification basins Page 1 of 22 • oxidation ditches • secondary clarifiers • return activated sludge • tertiary deep bed sand filters • chlorine contact basin • belt filter press for biosolids dewatering Un-disinfected tertiary effluent can be diverted to onsite disposal ponds, and chlorine disinfected tertiary effluent can be reused as plant water or in the community as non-potable water. The remaining tertiary disinfected effluent flows over a cascade aerator prior to discharge into Marsh Creek. Sludge is dewatered and trucked to a local landfill as daily cover material. EXISTING TERTIARY SAND FILTERS The existing tertiary filters are constructed around the Tetra Technology Underdrain Block system. There are three duty and one standby deep bed sand filters each installed in 54’-0” long, 9’-6” wide, 18’-1” deep above ground concrete tanks. The Phase 1 design flows are 5 MGD Average Dry Weather Flow (ADWF), 7.5 Average Dry Max Month (ADMM) flow, and 10.0 MGD Peak Wet Weather Flow (PWWF). The Phase 2 design flows are double that of Phase 1 or 10.0 MGD ADWF, 15 MGD ADMM, and 20.0 MGD PWWF. The Phase 1 facility has been constructed with provisions for the Phase 2 expansion including pumps, piping, valves, pipe penetrations, concrete tie-rods and break away sections for future tanks, electrical supply, controls, and instrumentation. The Phase 2 expansion will include the addition of four additional concrete tanks and deep bed sand filters. Secondary effluent (SE) from the conventional circular secondary clarifiers flows by gravity to the secondary effluent wet well and pump station. Alum and polymer can be mixed and injected into the secondary effluent stream to aid with coagulation and settling within the tertiary sand filters. No alum or polymer is being added at this time. Secondary effluent is gravity drained through a 6-foot deep bed of sand filter media, 1’-6” deep bed of layered gravel, and then through the Tetra Technologies underdrain blocks. The plastic underdrain blocks are hollow with screened tops to retain gravel and sand above and open on the bottom to let filtered effluent drain below. Filtered effluent (FEF) from the filter underdrain flows by gravity through the effluent box to the chlorine contact basins then out to onsite effluent disposal ponds, surface water discharge, or reuse. A portion of the filtered effluent overflows the effluent box to fill the clear well. Submersible backwash pumps located in the clear well are used to intermittently reverse flow Page 2 of 22 upward through the gravel and sand media filter bed to remove and waste particulates trapped in the sand media. Air from the backwash blowers is piped into the hollow underdrain blocks in conjunction with the up flow from the backwash water to scour and lift particles from the gravel and sand media. The air helps to roll the sand media and lift the waste particles to the water surface where they will overflow the filter weirs and gravity flow to the mud well. Motorized valves on the secondary effluent, backwash, and air lines are used to switch between filter and backwash cycles. The added air increases the effectiveness of the backwash cycle and reduces the volume of backwash water. Submersible pumps in the mud well send the dirty backwash waste (WW) back to the plant inlet screen. Each of the four sand filters are backwashed every other day. Two backwash cycles are run each day. A typical backwash cycle is started with the air blowers lifting the media and suspending the solids for a set time (currently 10 minutes) prior to the backwash pumps running. One backwash pump (rated at 2,900 gpm) pumps at 2,600 gpm for 25 minutes for a total daily backwash flow of 127,600 gallons. The backwash is wasted to the mud well and returned to the WWTP headworks for retreatment. Equipment supporting the Phase 1 tertiary filter process includes the following: • 2 each – 200 hp, 2,540 scfm positive displacement backwash air blowers • 2 each – 40 hp 2,900 gpm backwash (clear well) pumps • 2 each – 20 hp, 1,000 gpm waste (mud well) pumps No additional blowers or backwash pumps are required for the ultimate Phase 2 design flows so it is assumed that there is one duty and one standby pump for each application. The air blower has the largest electrical load in the plant. The filter backwash cycles are set to run on a timed schedule rather than on increasing headloss and water surface elevation through the filters to ensure that the air blowers are operated only during the off-peak electrical demand times of the day. The blowers are the single largest electrical load in the plant and at 200 hp each they increase reserve demand charge and billing tier rate fee for the plant. The City Operations Group estimates that the fees paid to reserve power for possible peak electrical demands of the plant and the cost associated with the increased electrical usage rates caused by moving to the next higher billing tier have an approximate annual cost of $75,000 per year. Page 3 of 22 The backwash waste pumps are sized to empty the mud well quickly in case there is a plant upset and the filters need more frequent or extended backwash cycles. For normal flow conditions the waste pumps are throttled down using the discharge valve to meter flow back to the headworks and a slower rate over a longer period of time. The backwash waste pumps are constant full speed pumps. A reduction in the volume of backwash water generated and air blower size by switching to another type of tertiary filter should reduce the plants monthly electrical use. One of the sand filters was taken off line for inspection and repair approximately 12 years after start up. It was deemed easier to dispose of the old sand media and replace with new rather than return the used sand media back into the filter basin. The cost to replace the sand was $80,000. There has been no need to replenish sand in the filter basins due to wash-out nor replace sand due to fouling or lack of performance. There is currently no need or schedule to replace the existing sand media. CLOTH TERTIARY FILTERS Operations staff at the City of Brentwood WWTP are considering the design parameters and costs of installing cloth media filters. The use of cloth media filters may decrease the backwash demand and wastewater sent back to the headworks for retreatment, and may reduce or eliminate the energy used to operate the backwash air blower. Cloth media filters may be installed instead of sand filters in the Phase 2 expansion and may be used to replace of the existing Phase 1 sand filters. TYPICAL OPERATION OF CLOTH FILTERS In a cloth media filter the secondary effluent is gravity fed or drawn by a suction through the media. Cloth media filters come in both inside-out flow and outside-in flow variations. The effective pore size of the cloth media reduces as the filter is used due the buildup of particles on the filters surface builds. When the water starts to move slowly through the media the water surface will rise. In a gravity fed filter, particulates continuously collect on the cloth filter surface which slows the flow through velocity and filtering rate, increases headloss, and raises the water level in the basin. The water level will rise to a control set point that will start a cleaning or backwash cycle. Typical cleaning systems either vacuum the filter surface, or use pressurized backwash spray. The backwash water is wasted to the treatment plant headworks. Page 4 of 22 Some models of cloth filters are made specifically to be retrofitted into concrete sand filter basins. There are some cloth filters that fit nicely into sand filters equipped with a traveling bridge where the traveling bridge is reused to clean the media. Some modifications to the existing concrete tanks may be required to install a packaged cloth media filters.