Assessing Performance of Interface/Sludge Blanket Level Analyzers

The District of Columbia Water and Sewer Authority (DC WASA) Blue Plains Advanced Wastewater Treatment Plant (AWTP) is the largest wastewater treatment facility of its type in the United States with a rated annual average capacity of 370 mgd and a peak wet weather capacity of 1.076 billion gallons per day. The plant is restricted on its discharge by its NPDES permit to a Total Phosphorus of 0.18 mg/l and a Total Nitrogen goal of 7.5 mg/l. Additionally, Blue Plains WWTP treats combined sewer flows where during a rain event flows can vary from a low of 240 mgd to a high of 1,076 mgd in a matter of hours. Control of solids inventory is critical for the plant. Tracking the changes in solids inventories and sludge blanket levels is critical for treating the flows and not losing solids due to washouts.The Blue Plains AWTP has a total of 106 sedimentation basins that are on-line at all times. The plant has 36 circular primary clarifiers, 60 rectangular activated sludge sedimentation basins, and 10 circular gravity thickeners. Blue Plains has a two stage activated sludge system consisting of a short sludge age, high rate, first stage, followed by a combined nitrification-denitrification secondary stage that uses methanol as a carbon source for denitrification. The effluent from the second stage is treated by multimedia filtration prior to discharge into the Potomac River. Phosphorus limits are achieved by using two point iron salt addition in the Primary and Secondary (first stage activated sludge) processes.Presently, sludge blanket readings are recorded once each shift for each activated sludge clarifier for a total of 180 manual readings taken over three shifts. During a washout condition or in response to equipment malfunction, these readings are taken more frequently on individual tanks where problems are observed. The plant is staffed 24 hours a day with a three shift rotation. The sludge blanket data collected by a variety of operators using a sludge judge is susceptible to varied interpretations as to the location of the water/sludge interface by individuals working under differing lighting conditions. DCWASA is in the process of evaluating options for minimizing the effort associated with this task while improving reliability of the data.DC WASA partnered with the Instrumentation Testing Association (ITA) to conduct a cooperative performance evaluation field test of online interface/sludge blanket level (ISBL) analyzers. DC WASA and other ITA members had a keen interest in the ability to continuously monitor sludge levels in water and wastewater treatment clarifiers using online ISBL analyzers so they can benefit from having more timely information for enhanced process control and reduce labor requirements. This cooperative test project was supported by DC WASA and other ITA members representing treatment facilities, industry, consultants and manufacturers including Dow Chemical, Metropolitan Water Reclamation District of Greater Chicago; City of Houston; City of Yuba City; EMA, Inc.; Entech Design, Inc.; Hach Company; Royce Technologies; and Solartron Mobrey.This paper provides a background on the plant's experience with sludge blanket control, and an overview of ITA's testing approach starting with committee development of test protocol and procedures, design of test site reference measurements, test site personnel training, analyzer installation, calibration, operation, maintenance requirements, and general information of how typical ITA test data results are presented in a report format.The performance evaluation field test performed at DC WASA Blue Plains AWTP introduced reallife circumstances such as power outages, changes in personnel, and climatic variations including exposure to hurricane Isabel. The test project not only recorded instrument performance and maintenance requirements but also noted how the analyzers held up to everyday "real-life" situations that are difficult to account for with laboratory/pilot testing.