Wastewater Treatment Plant Design and Rehabilitation
Regional Wastewater System Evaluations
When additional customer cities are added to a regional wastewater facility, or when the choice is between a plant expansion or resurrecting a mothballed WWTP, determining the best course of action can be daunting. Our team recently assisted a major regional wastewater service provider with such decisions at two separate regional systems. In both cases, we coordinated flow and rainfall monitoring and associated inflow/infiltration analyses and development of future average and peak flow scenarios. Population and flow projections were considered, as was the cost of returning old facilities to service. In one of these systems, the owner ultimately elected to expand an existing treatment facility from 1.5 MGD to 3.0 MGD; Perkins Engineering Consultants, Inc. (PECI), served as prime consultant for design of the plant expansion, which is now in full operation. In another, PECI conducted a subsequent study evaluating adding two new customers in the regional system.
Nutrient Removal Evaluation and Aeration System Improvements
PECI evaluated biological nutrient removal (BNR) process configurations for a 33 MGD regional wastewater treatment plant. Historical operating and additional sampling data were collected, organized and analyzed to define design conditions, including diurnal fluctuations. Unit processes, equipment sizes and configurations were determined for three BNR processes and preliminary layouts were developed for each configuration. Life cycle cost comparisons were prepared for each of the three BNR alternatives.
The first 13.9 MGD phase was constructed at a cost $4.5 million. Improvements included replacing three existing multistage centrifugal blowers with high speed turbo blowers (6,000 scfm each) and addition of a new CMU electrical building. Baffle walls, anaerobic and anoxic zone submersible mixers, mixed liquor recycle pumps and piping, and new effluent gates were added to six existing Aeration Basins.
Regional Treatment Plant Clarifier and Thickener Rehabilitation
PECI served as a subconsultant responsible for design of mechanical and process improvements to a 152 MGD treatment facility serving over 20 cities. This multifaceted project involved a large design team, in which PECI led design efforts for:
New primary clarifier scum concentrators;
Mechanism replacement on four 75' x 275' rectangular clarifiers;
Mechanism replacement on circular gravity thickeners:
Aluminum truss supported flat covers on the gravity thickeners; and
Corrosion and coatings repairs on multiple structures.
The overall project bid at $23 million and was completed in 2015.
Nutrient Removal Evaluation
PECI identified and evaluated nutrient removal process configurations for a 16 MGD municipal wastewater treatment plant. Unit processes, configurations and preliminary costs were determined for chemical phosphorous removal and biological nutrient removal (BNR) processes. Three alternatives for BNR were evaluated as part of the project. Capital and operating costs for one alternative for BNR, Anaerobic/Anoxic/Aerobic, along with upgrading the aeration system, chemical phosphorous removal and side stream treatment of the supernatant stream from solids handling were also determined.
Regional Treatment Plant Hydraulic Improvements
PECI serving as a subconsultant to another firm, was responsible for identifying hydraulic relief improvements needed to enable this 152-MGD regional plant to better distribute flows from its 12 secondary clarifiers to its 32 low-head effluent filters, some of which are being abandoned while others are receiving significant capacity upgrades via cloth media retrofits. Two relief tunnels were identified and designed. The first involved a relatively short but complex 84-inch relief tunnel to connect the effluent troughs from two isolated banks of clarifiers. This pipeline was partially installed by open cut, but due to electrical duct banks and other large utilities in the vicinity, specific portions had to be installed by tunneling in tight, confined operating plant areas and beneath roadways.
PECI has designed multiple improvements to a 10 MGD plant serving two cities in north Texas. PECI specified a biological odor control system (bioscrubber) with organic media for the influent lift station. This allowed the owner to reduce the usage of the existing chemical scrubber located at the solids handling building and resulted in substantial chemical and energy costs. To reduce corrosion in the existing electrical room, PECI specified a positive pressurization unit to pressurize the room with scrubbed air. In addition PECI redesigned the existing chemical storage area with new bulk tanks to accommodate full truckloads of sodium hydroxide and sodium hypochlorite.
Primary and Secondary Clarifier Rehabilitation
PECI served as prime consultant responsible for design of major corrosion repairs to four large primary clarifiers and four secondary clarifiers at a 48 MGD regional wastewater system serving multiple north Texas cities. The project involved assessment and repair of corrosion damage to concrete structures, rehabilitation of four 135-foot aluminum low-profile basin covers, weir and baffle replacements and skimming system rehabilitation. An assessment of the effectiveness of the scrubbing and ventilation systems serving the primary clarifier was also included.
Blower and Aeration System Upgrade
PECI served as prime design consultant assisting a major north Texas city with an update of the activated sludge process in its 24 MGD wastewater treatment plant. The project included installation of new high speed turbo aeration blowers, air piping, and fine bubble membrane diffusers. The existing aeration system included four multistage centrifugal blowers. The project was placed in service at a cost of $2.6 million.
Snail Collection and Slurry Pumping System
As part of a larger plant rehabilitation project, PECI designed a special settling basin to capture snails from a trickling filter process. A pumping system was designed to move the snail-laden slurry into a classifier so that snails could be deposited in a roll-off container for hauling off-site. The project significantly reduced snail accumulation in downstream basins.
Screen and Conveyor System Replacement
PECI assisted the City of Garland with replacement of screening and conveyor equipment in the headworks building at the City’s 24-MGD Rowlett Creek Wastewater Treatment Plant. Equipment installation was performed by City personnel.
Plant-Wide Rehabilitation Improvements
PECI was retained by to assess and prioritize rehabilitation needs throughout a 24 MGD wastewater treatment plant serving multiple cities. The treatment facilities had been constructed and expanded under numerous contracts, with many facilities over 40 years of age. Corrosion and structural deterioration was a concern in a number of areas.
PECI served as prime consultant responsible for detailed design of the highest priority improvements under two construction projects. These projects included extensive corrosion repairs to a number of covered concrete structures within the plant, drainage improvements inside the plant’s dewatering building, addition of a 5,000 CFM bioscrubber for odor control, replacement of the plant’s aging return and waste activated sludge piping and distribution system, rehabilitation of approximately 3,500 square yards of in-plant paving surfaces, and addition of access and security improvements. Full-span truss supported aluminum covers were added to the plant's primary clarifiers, screens were replaced, and major electrical gear was rehabilitated.
Plant Rehabilitation Needs Assessment
PECI served as prime consultant responsible for a plant-wide assessment and prioritization of rehabilitation needs for this 152 MGD regional wastewater treatment facility. The effort included review of the age, criticality, and condition of dozens of major process units, along with development of projected costs for rehabilitation. Coordination with multiple consultant teams was required. The evaluation identified recommended in-plant rehabilitation totaling approximately sixty million over a span of six to ten years.
Wastewater Treatment Plant Domes and Mixing System Addition
Perkins Engineering Consultants, Inc. assisted the City of Sherman with addition of domes to cover three 90-foot final clarifiers for algae control. The project also included addition of a jet mixing system for the plant’s wet weather storage facility. The project was placed in service at a cost of $1.4 million.
Vehicle Washing Facility for Biosolids Transport Trucks
PECI designed a trailer washing facility to clean emptied biosolids trailers at a regional disposal facility. PECI evaluated possible options and develop cost estimates for the following two key facets of design:
Truck wash location; and
Water clarity and disposal.
Four truck wash locations were evaluated. Options for increasing water clarity included chemical coagulation, electro-coagulation and water softening. The disposal facility is not served by an existing sewerage system and recycle of flows was considered.
Wastewater Treatment Plant for Retail Development
PECI, serving as a subconsultant to another firm, was responsible for drafting technical specifications for the preselection of equipment for a 0.014 MGD package wastewater treatment plant in Hayes County, Texas. PECI led pre-selection of a plant equipment manufacturer, then preparation of plans and specifications for the construction of the treatment system. PECI was the design lead for the effluent storage tank that would serve as storage for the subsurface irrigation system. PECI will be assisting with construction services and plan submittals to four state, regional and local agencies for approval.
Truck Weigh Scale Evaluation
Working with multiple departments, PECI evaluated on-site weighing options at several wastewater treatment plants to assist our client with maintaining compliance with local, state and federal transportation rules. Reviewing container loading and hauling operations, analyzing sludge processing data, and current documentation, PECI evaluated various weighing options including portable scales, steel deck scales, low speed weighing scales, electronic or hydraulic load cells on the existing container systems, and overhauling the existing sludge conveying and receiving equipment with cake storage systems. Conceptual layouts and preliminary costs were developed for each of the options presented. In addition, modifications to operational procedures were included that could be performed without capital expenditures to maintain compliance with transportation rules.
Regional Treatment Plant Effluent Filter Rehabilitation
PECI is serving as the prime consultant to retrofit existing traveling bridge sand filters with cloth filters for a 24 MGD wastewater treatment plant. The project consists of modifying the filter basins to accommodate the proposed filters, installing new influent weir gates, drain and yard piping modifications, and a structural canopy to cover the filter basins. The project site is subject to potential flooding, so additional design elements such as shut off switches, equipment enclosures, and raised control panels were implemented to protect the equipment should flooding events occur. This project is currently under construction and expected to be finished in first quarter 2022.