Marshalltown, IA

Biosolids Improvements

  • Client City of Marshalltown
  • Type Wastewater
  • Services Evaluation, Design, Construction
  • Construction Cost $5.3 Million (2004)
  • Project Manager Dave Fox, P.E., BCEE
  • Project Engineer Keith Hobson, P.E., BCEE; Lance J. Aldrich, P.E., BCEE

CHALLENGE

Since 1939 Marshalltown has treated its wastewater at the same location; as part of the treatment process, inorganic and organic material commonly known as sludge is removed from the raw wastewater. It is treated and eventually land-applied as fertilizer on field crops. After sludge is treated, it is commonly referred to as biosolids.

FOX Engineering’s relationship with the city began in 1980 with a significant renovation. Thirty years goes by quickly. In 2000 the city was faced with failing anaerobic digesters. Significant improvements were needed, particularly in the area of sludge handling and treatment. The facility is equipped to utilize the methane gas from the anaerobic digestion process for heat and electricity, but the digester covers were leaking a large portion of the gas, digester heating was inefficient, and mixing was inoperable. The goal was to utilize the anaerobic digestion process to its fullest extent to maximize the use of the methane gas product. At the same time, it was a desirable objective to improve the quality of the final biosolid product.

Heat exchangers

Heat exchangers

Setting up for placing of covers

Setting up for placing of covers

 

SOLUTION

The city hired FOX in 2000 to evaluate the sludge facilities, recommend improvements, and provide budget costs. The pre-design planning process involved preparation of project concept; identification, development, and evaluation of different project options; and assistance with obtaining funding.

All three existing anaerobic digesters had new steel covers installed. One digester was converted to a TPAD system, one remained as a standard mesophilic anaerobic digester for standby service, and the third was retained as a secondary digester for sludge storage and gas collection. The digester converted to TPAD has a 42-foot diameter tank created inside the original 75-foot diameter tank. This divides the tank for the two temperature phases needed, the center tank for thermophilic digestion (operated at 131 degrees F) and the outer ring for mesophilic digestion (operated at 95 degrees F). A pair of progressing cavity pumps transfers the combined primary and secondary raw sludge through the spiral sludge-to-sludge heat exchangers where the sludge is preheated and moves on to the thermophilic digester.

As the raw sludge is pumped into the thermophilic digester, another pair of progressing cavity pumps removes sludge from the thermophilic digester, pumps it through the other side of the sludge-to-sludge heat exchangers where the thermophilic sludge is cooled to mesophilic operating temperatures, and then transfers the cooled sludge to the mesophilic digester. Insulation of the digesters has been increased and efficient mechanical mixing was added.

Other portions of the project included installation of new natural gas boilers to produce heat more efficiently than the existing engine generators when methane gas is not available. Sludge thickening systems were modified to allow continuous automated operation. In addition to biosolids improvements, the final project involved upgrades to heating, ventilation, and controls.

The raw wastewater pumps were replaced with equal capacity but slower rotating pumps to decrease wear from grit. Controls were added to automate and monitor the remainder of the treatment facility which had not previously been included in the plant-wide control system.

 Replacing digester cover

Replacing digester cover

Progressing Cavity pump under construction

Progressing Cavity pump under construction

OUTCOME

In June 2004 improvements were operational. FOX provided all services for onsite construction observation, inspections, site visits by project engineers, review of contractor submittals, responses to requests for information, and processing of change orders. FOX also consulted with the WPCP staff on start-up and operational issues and training.

This unique project significantly increased production and utilization of methane gas from the anaerobic digestion process, significantly reducing the amount of natural gas and electricity required to operate the WPCP. The biosolids produced are of higher quality (Class A) and allow the City greater flexibility in finding locations for their disposal. The control system allows an operator to monitor the entire facility from anywhere at the facility, potentially allowing a reduction in operating labor.

New Sludge Storage Tank - starting on the fourth ring

New Sludge Storage Tank – starting on the fourth ring

Gas bubble storage

Gas bubble storage