TURN SLOWLY TO BETTER DEWATERING: CASE STUDIES WITH ROTARY PRESSES

Sludge and biosolids dewatering is a critical element in most wastewater treatment facilities solids handing schemes. Proper dewatering can significantly reduce the cost for further handling, treatment, or hauling of the cake. Efficient dewatering requires minimal manpower and chemicals. Recycle streams that are low in conventional contaminants will also reduce the cost of treating these liquids. Selection of the dewatering equipment needs to consider all of these factors to result in a successful, low cost dewatering system.This paper will present case studies at three different wastewater treatment facilities. The Scarborough Sanitary District is expanding the capacity of their wastewater treatment facility to handle average flows of 2.5 mgd, and peak flows of 7.98 mgd. Part of this upgrade and expansion project is to revitalize its aging solids processing system. During project planning a detailed life cycle cost evaluation was conducted to select treatment and solids handling system components used for the upgrade. The Solids Dewatering options which were evaluated included:Refurbish existing standard ( 8 roll) Belt PressesInstall self-enclosed heavy duty, high solids (14 roll) Belt PressesInstall CentrifugesInstall Rotary PressesData from on site testing was input into a cost model, and the rotary press was selected by the District for the upgrade and expansion due to its enclosed arrangement, low operating speed, and simple operating requirements.The City of Charleston (Charleston Water System-CWS) was faced with a similar situation several years ago at two of their wastewater treatment facilities. Charleston had belt presses which were over 20 years old as the existing dewatering equipment at the Plum Island plant. The options considered were as flows:Replace the existing belt presses with new state-of-the-art belt presses;Replace existing belt presses with a centrifuge with provisions for a second in the future;Replace existing belt presses with two rotary presses with provisions for two future units.Preliminary on-site pilot testing of the rotary press demonstrated that the press would dewater both a mixed primary and secondary sludge and a straight secondary sludge to solids levels comparable to a centrifuge and better than a belt press. After considerable research and the pilot testing, rotary presses were chosen to replace the existing belt presses. The rotary press was also tested for the Daniel Island plant and chosen to be installed as a part of a totally new dewatering facility at the Daniel Island facility.The rotary press, relatively new to the wastewater industry, is proving to be an excellent choice for many dewatering applications. This paper will provide a description of the rotary press equipment and present the results of evaluations of several installations of rotary presses. Pilot testing, performance testing, and operating data will be presented. A summary of some of this data is provided below. The data will demonstrate that a rotary press is a viable alternative to standard belt presses, newer heavy-duty high solids belt presses, or centrifuges.