Twenty years after the last rebuild of the anaerobic digestion system at the City of Massillon’s wastewater treatment plant, this system was barely meeting minimal requirements.  As a result of these problems, the City was forced to purchase large amounts of natural gas for digester heating.  CTI was retained to design improvements to the sludge digestion facilities as part of a larger contract to expand the overall WWTP.
CTI’s design included the following elements:

• Replacement of gravity thickeners with gravity belt thickeners: This measure made it possible to double sludge feed concentration (from 3 to 6 percent solids), increase hydraulic detention time, and improve digester performance.
• Replacement of sludge recirculation pumps and tube-in-tube heat exchangers: As a result, more sludge contacts the hot water side of the exchanger and temperatures remain consistently high, improving bacterial decomposition.
• Replacement of fixed digester covers with new floating covers: The new covers allow greater operational flexibility and restore design functionality.
• Expansion of 12.5-mgd secondary facility to 15.8-mgd advanced secondary facility: Aging components were replaced with state-of-the-art equipment to comply with new regulations and to meet future demand.

The success of these improvements was apparent almost immediately.

• Average volatile solids reduction increased from 60.4 to 73.2 percent.
• The City used 31 percent less natural gas in the 8 months after construction versus the 8 months before.
• The City saves approximately $60,000 per year in natural gas costs, based on current prices.  (Prices had increased 275 percent in 3 years prior to start-up of the improvements.)
• Captured digester gas, rather than natural gas, is used to heat the digesters.  In the near future, digester gas will also be used as fuel for heating boilers for the on-site buildings.

CTI SERVICES

• Planning
• Design
• Drawings and Specifications
• Bidding Assistance
• Construction Administration
• Resident Project Representation