Sludge Handling Problems and Solutions For The Village Of Oak Harbor

By: Bob Lulfs, Orwa Circuit Rider and John Liske, Oak Harbor WW Superintendant

This article details the history, problems, and solutions that the village of Oak Harbor faced in order to overcome their sludge disposal issues.

The Village of Oak Harbor Wastewater Treatment Plant was built in 1958 as a primary treatment facility. In 1975, to meet higher effluent quality requirements, the plant was expanded with trickling filters as a secondary means of treatment. The collection system is 75% combined and 25% separated. Dry weather design flow is 0.735mgd with a wet weather design flow of 4.33mgd. At that flow, 2.16 mgd receives secondary and 2.17 mgd receives only primary treatment. All flow is sent through ultraviolet disinfection prior to entering the Portage River.

The original plant included 2 - 80,000 gallon anaerobic digesters and four sand drying beds; each one is 20 x 60 foot to meet the sludge demand. These were left unchanged during the plant's improvements completed in 1975 and in 1990.

A 65 x 95 foot building was erected over the beds in 1990 to increase drying rate on the beds. While somewhat successful, it did not prove to be the answer to the problem. Too much sludge was being produced. Most, if not all of the sludge, was still being sent out in a liquid form by tanker trucks. This worked as long as the farm fields, used for sludge application, were available. Many times this was not the case due to weather conditions, sludge hauler availability or un-harvested crops still in the field.

The village decided to seek an affordable alternative to liquid sludge application. Consideration was given to several different options:

  1. Re-building the drying beds. While this may have helped, the beds were still too small to address the current and future sludge production. Also since the beds had been in use since 1957, it was felt the under drains were plugged.
  2. Add a large sludge holding tank and send the sludge out by truck. The drying beds would no longer be utilized. This did not address the problem of weather conditions, farm field availability, or the sludge hauler contactor's scheduling.
  3. Numerous options, including vacuum filter beds, centrifuges, and belt presses, were studied. The belt press was the most desired option but appeared too costly.

After three years researching the options, an affordable belt press was found. The question now became what other changes or improvements were necessary in the plant for the use of this option.

Upon inspection of the existing anaerobic digesters, it was found that the floating covers needed to be replaced. The cost of replacing the covers was extremely high. The village's engineering firm recommended a complete change in the sludge treatment process. Their recommendations were to remove the covers, drain and clean the tanks, remove all piping and convert the digesters to aerobic digestion.

In addition, a third tank was to be erected. This tank was to be a 90,000-gallon glass lined steel tank.

This additional tank was needed to achieve the storage capacity set forth in Ten State Standards, a 120 to 180 day holding capacity.

One of the reasons for the switch to aerobic digestion was that it is just as cheap, if not cheaper, to go to aerobic digestion than to repair or replace the floating covers. Another reason was that the belt press that was to be used worked better with an aerobic sludge. This press was to be a 1.5-meter single belt of stainless steel construction.

Reasoning for the use of a belt press included:

Affordability of the unit. We are a small plant with limited funds. This press was less costly than other units. The press was designed for small industrial use; however, the company has found it works very well at small wastewater plants. They have, been actively moving in this direction.

Compact size of the unit. It had to fit into the existing building that the village had installed over the drying beds. This building had been built for $70,000 in 1990 and was in excellent condition. To not use it, would have been a waste of village funds.

  1. The belt press was to be delivered to the plant on a skid. All equipment was mounted on the skid with no setup required by a contractor. This meant lower installations costs. The skid also allowed the belt press to be rolled into place through the doors of the existing building.
  2. Stainless steel construction of the belt press at a low cost. This unit will give a great deal of durability at an acceptable cost to the village. The unit cost of $90,000 is far less than other belt presses the village had seen prior to this.
  3. This unit is simple to operate and maintain. Operators had no prior experience running a belt press. Because this is a small plant, an operator was not going to be available to watch the press at all times. This unit runs by itself and only requires a checkup every 30-45 minutes.
  4. Easy clean up after operation. Because of the limited manpower at the plant, a belt press unit was necessary that had a fast easy startup, simple operation, and fast cleanup. This unit seems to answer all those needs.

The sludge handling changes were completed in 1999-2000. The project cost was $1,000,000, and included the removal of the digester floating covers, demo of the sludge heating boiler, erection of a third digester tank, and re-habilitation of the sludge drying beds and the building that was over them. Changes to the sludge drying beds and building included:

  1. Removal of the sand drying beds, all concrete walls, and driveways.
  2. A concrete floor throughout the entire 65x95 foot building.
  3. Creating 3 separate areas in the building.
    1. The belt press room is 29x44 foot. It is large enough to hold the 1.5-meter press and a future expansion, be it another press or something else.
    2. A second area was a 22x28 feet, room to house four blowers for the new aerobic digesters. These units are sound proofed and run very quiet.
    3. The sludge storage area for the pressed sludge is a 63x63 foot room. The village has purchased a skid loader to move the pressed sludge around in this area as well as loading for removal. The sludge is land applied by a private contractor. There is about 5 months of storage in this area.

The belt press unit has been in operation since February 2000. It has exceeded all expectations. Inlet concentrations are between 1-2% solids and final cake is 15-18% depending on rate of application. We run the press one day a week at the present time. The operation takes about 15 minutes to startup. The press runs unattended with an operator checking it every 30-45 minutes. Shut down and cleanup takes about 30 minutes. This is about half the time a larger press takes to clean up. The unit has pressed about 100 dry tons in the ten months of operation.

The Village of Oak Harbor is very satisfied with the new sludge handling facilities. The plant operators are able to process the sludge on site anytime it is necessary. Overall operation of the plant has improved because tanks can be cleaned or repaired at any time. No longer do we have to depend on the weather becoming a factor. We no longer have to depend on a fixed schedule of removal by the private contractor.

reprinted from Ohio Rural Water Association "Downhill Flow" magazine January 2001