Innovative Sludge Lagoon Closure Saves Money and Provides Peak Flow Storage System

The City of Dallas Water Utilities Department (DWU) faced two major challenges at its Central Wastewater Treatment Plant (Central Plant) in 1998. First, an existing sludge storage lagoon at the Central Plant was now inactive and was required to be officially closed. When the existing lagoon was closed, DWU would require a facility for the emergency storage of approximately ten million gallons of sludge. Second, a Major Interceptor Study indicated that additional peak flow detention was required at the Central Plant. The existing peak flow storage at the Central Plant was approximately 65 million gallons (MG) and a minimum of 215 MG detention capacity would be required to accommodate the additional flows associated with a five year design storm event.The City's Residuals Wastewater Master Plan had concluded that the only alternative for closing the Lagoon was to remove the sludge from the lagoon and haul it to a hazardous waste landfill, at an estimated cost of 12 million. This did not include the costs of improvements required to convert the lagoon to a peak flow storage basin. It was clear that the peak flow improvements were required and due to space limitations at the site, peak flow improvements needed to be constructed on the sludge lagoon site.Detailed investigations were implemented to identify the cost effective alternatives available for managing the sludge in the lagoon. Approximately 225,000 cubic yards of sludge varying in consistency from solid to liquid was in the lagoon. Several alternatives for the ultimate disposa of the sludge were evaluated. Alternatives for sludge disposal considered included: co-disposing the sludge off site at City-owned facilities, contract treat and land apply for beneficial re-use; contract haul for disposal; and haul and land apply with City forces. All alternatives were carefully evaluated and measured against the DWU Residuals Master Plan, current City operations and capabilities, state and federal regulations and economic factors to determine the best Sludge management solution and also provide the required Peak Flow Detention System to meet the storage need identified in the master plan.Using creative thinking, an innovative “outside the box” solution was developed which included moving the existing sludge to one end of the lagoon, isolating it with a geotextile reinforced containment berm, installing a cap and cover and closing the sludge cell in place. The remainder of the lagoon was then converted for use as a peak flow storage basin.This Paper and Presentation will discuss:The innovative Sludge Lagoon Closure ConceptThe challenges and the results of the Site Assessment and Waste Characterization of the material in the sludge lagoonThe regulatory coordination necessary to arrive at the final Closure PlanThe unique design and construction challenges involved in relocating and containing the sludge and restoring the lagoon for use as a Peak Flow Storage BasinThe cost savings and peak flow storage benefits recognized through using this innovative closure option compared to conventional removal and disposal optionsSome of the innovations and major elements of the project included:Special equipment and sampling techniques were required to access the remote portions of the lagoon and retrieve samples for the Site Investigation and Waste Characterization. The environmental characterization was successfully completed and classified the sludge as a non-hazardous material. The sludge did have some elevated concentrations of metals that precluded agricultural land application and some other options as potential ultimate disposal solutions.An innovative, on-site Closure Plan for the sludge storage lagoon was developed through close communication and coordination with the Texas Natural Resource Conservation Commission (TNRCC) (now the Texas Commission on Environmental Quality (TCEQ)), the state regulatory agency. A resourceful, carefully thought-out closure plan allowing on-site closure of the lagoon was prepared and presented to the TNRCC. This plan saved millions of dollars over the planned off-site closure cost. The Closure Plan demonstrated to the TNRCC that on-site closure with no additional RCRA-type lining or leachate collection systems was a viable alternative, saving DWU additional closure costs.The variable consistency of the sludge material presented numerous challenges during the preparation of design documents and implementing the actual construction of the project. The sludge varied from a soil like material with a vegetative cover on the surface of the lagoon, to a watery liquid at the bottom of the lagoon. The design incorporated the use of a variety of different geotextile materials to ultimately contain the sludge and allow for the difficult materials handling tasks.The existing available peak flow detention volume was expanded from approximately 65 million gallons to an approximate peak flow detention volume of 215 million gallons. The Peak Flow Detention System design incorporated multiple peak flow management schemes that maximized flexibility in the hydraulic operation and performance. The final peak flow detention system design incorporated three separate basins which operate as settling basins in series. This design simplifies clean up maintenance after a storm event requires the basins to be used. It also provides the potential opportunity to apply innovative treatment processes options in the future to a dilute wastewater that has gone through a significant pre sedimentation treatment process. That is, it may be possible to return the dilute, partially treated wastewater to a later stage in the treatment process train rather that back to the head of the plant. This could provide additional cost savings in the future.DWU also desired to have an Emergency Biosolids Detention facility in the event that the planned biosolids pump station and force main between the Central Plant and the Southside Plant failed. The creative design also allowed for approximately ten MG of the current detention volume to be allocated to emergency detention of biosolids as part of optional plant improvements.In addition to the cost savings associated with the on site Closure, substantial additional cost savings were recognized through Value Engineering on the project.