An Evaluation of Filter Underdrain Failure and Lessons Learned

The city of Houston Northeast Water Purification Plant (NEWPP) was constructed in two 151,400 cubic-meter-per-day (40 million gallons per day (mgd)) phases with a total of sixteen filters. The Phase I Filters were constructed between 2002 and 2004 and placed into service in 2004. The Phase II Filters were constructed in 2005 and placed into service in 2006.Recently, the plant has had two underdrain failures at the two of the Phase I filters. An inspection was performed on the two filters to identify the causes of the underdrain failures. Samples of grout between the plastic underdrain blocks were collected for laboratory analyses, including petrographic analysis, compressive strength, water-soluble chloride content, and sulfate content. The results of the laboratory tests are discussed and water treatment chemistry, filter performance and backwash operation, and filter hydraulics are reviewed to determine their roles in the grout deterioration.