Controlling Hospital Grease, Sludge Discharges

Sept. 3, 2002

About the author: Worldstone, Inc. manufactures separation tank monitoring solutions for on-site wastewater management and is based in Sandwich, Mass.

One of the major problem areas in St Petersburg, Florida has two hospitals and a nursing home that discharge to a common sewer line. Historically, the city of St. Petersburg had to clean this section of the sewer line at least four times a year to avoid blockages and sanitary sewer overflows. From time to time, the city even had to jet out the line on a monthly basis.

The location of the tanks, the grade of the sewer line, and the timing of the sampling made it difficult to identify the source of the problem. Meanwhile, the hospitals had their 3,000-gallon grease traps pumped every three weeks.

Working together, the city and one of the hospitals took action to control the discharge of grease from the hospital's facilities and to reduce the costs of maintenance for both the city and the hospital.

Wastewater utilities in this area are especially proactive about controlling discharge of fats, oil, and grease into wastewater collection systems. Florida and the city of

St. Petersburg are part of EPA Region 4, made up of eight southeastern states. At present, the EPA is conducting a voluntary Capacity Assurance, Management, Operations, and Maintenance (CMOM) program in Region 4. Region 4 EPA has set a goal of zero SSOs by 2011.

Most blockages and sanitary sewer overflows in wastewater collection systems are caused by grease. Therefore, St. Petersburg places great emphasis on programs to control grease discharges.

The hospital installed GREASEwatch, an automated separation tank monitoring system from Worldstone, Inc., in Sandwich, Mass., in their on-site wastewater system. The system provides continuous monitoring of the hospital's grease interceptors/traps, tracking the grease, sludge, liquid level, and temperature in the tanks. Once in operation, the monitoring system showed that, intermittently, the temperature in the tanks reached unacceptably high levels far in excess of what the city ordinance allowed.

Neither the hospital nor the city had previously been able to detect these intermittent spikes in temperature. Further investigation showed that, periodically, the hospital kitchen staff was pouring 250 gallons or more of boiling water into the floor drains. The kitchen staff were unaware that the drains were connected to the grease interceptors. The boiling water prevented grease from separating from the liquid before discharging into the sewer line.

The hospital launched an aggressive program to educate and train kitchen staff in good waste disposal practices. At the same time, continuous monitoring of the hospital's onsite wastewater system verified for the hospital and the city that the temperature in the tanks now stayed at acceptable levels consistently. Temperature spikes due to introduction of boiling water had been eliminated.

Going forward, the hospital tanks require less frequent pumping; now they are pumped every eight to ten weeks as opposed to every three weeks. The city monitors temperature and the levels of grease, sludge and liquid levels in the tanks. The city collections crew no longer has to service that sewer line on a regular basis.

Both the hospital and the city will save money. The hospital's expense for servicing the tanks has been reduced by more than half. The city reports savings on the costs of sewer line maintenance to eliminate blockages, as well savings on the costs associated with cleaning up overflows.

Previous Approaches

Previous approaches included manual inspection, sampling, pumping on a schedule and, cleaning out the sewer line every three months.

The City of St Petersburg sampled the hospital's wastewater at the point where it discharged into the sewer line. Because of the location and grade at the sampling point, and because of the timing of the tests, the sampling data was not very reliable.

The sampling was conducted during normal business hours. However, the introduction of boiling water into the wastewater system took place at the end of the evening shift around midnight. Therefore, the city's data gave no indication of the problem which existed.

In order to avoid sanitary sewer overflows, the city had to clean out this section of the sewer line at least four times a year and often even more frequently.

Worldstone systems utilize an ultrasonic transducer mounted on a pipe suspended in the middle of the effluent. The acoustic signals in the transducer monitor the levels of sludge at the bottom, grease at the top, and the liquid in the pipe mounted in the tank. The pipe acts as a stilling well, maintaining the liquid level of the tank.

Using these measurements and some preset tank dimensions, the positioning and thickness of the solids and level of the liquid within the tank can be calculated. The data recorded by the monitor is fed back digitally to a control unit. The control unit may be mounted outside in a weather-proof box or in the customer's building. Data from the control unit can be transmitted via modem to the Internet. Regulators can view tank status from their office.

Data is presented to the users at their desk in a format that is easy to understand. Programmable settings allow the service provider or the regulator to set when the tank should be pumped based on the actual amount of solids in the tank.

At any time, owners can push a button to find out how close the tank is to its next pumping based on the percentage of total allowable solids, top solids, or bottom solids in the tank. Status of the liquid level in the tank in inches, high or low relative to its normal level, also can be accessed at any at time via a button on the control unit.

Tank temperature is also available, which is important with grease traps and in very cold climates. Critical solids and liquid levels can be programmed into the system triggering an alarm or warning when exceeded.

The system measures solids, liquid, and temperature every 60 seconds, and it keeps 20 years of data in compressed format, serving as an historical record of changes in the tank over time, to document tank performance and routine maintenance.

Benefits of Remote Monitoring

The major advantage of the monitoring system is that it gives advance warning of problems and the need for service without manual inspection. With 24/7 monitoring, the hospital and the city of St. Petersburg were finally able to identify problems even when the inspector was not there. The monitoring system gives them a powerful tool to prevent blockages and sanitary sewer overflows and it provides documentation of regulatory compliance at less cost.

In addition, the monitoring system makes it possible to keep track of tanks that are not easily accessed or cannot be accessed during normal business hours. At many locations like theme parks, casinos, shopping centers, and malls, the traps can only be opened during a two to three hour period after midnight because of the smell.

In other locations, the grease interceptors are 18 ft. or more below ground, and they cannot be inspected visually or by standard dipping methods.

The monitoring systems require minimal ongoing maintenance. When they are first installed, the system has to be calibrated. In some cases, the pipe in which the transducer is mounted must be flushed right after installation to eliminate any solids or scum that were introduced during installation.

Between pumpings, the monitor is designed to be maintenance-free. After the tanks are serviced, it may be desirable to rinse out the pipe and transducer.

As for the hospital's grease interceptors, the maintenance and service requirements were cut in half.

Cost/Benefit Analysis

The city and the hospital anticipate pay back on their investment in less than one year.

The monitoring solution is highly effective for both parties relative to the alternatives of manual inspection, sampling, pumping on a schedule and cleaning out the sewer line every three months. Benefits to the city and the hospital are listed below.


  • Prevent backups, avoid costly fines and avoid negative publicity.
  • Verify that tanks have been correctly pumped and properly serviced.
  • Provide continuous feedback so the hospital can ensure that kitchen staff use best management practices for waste disposal.

City of St. Petersburg

  • Advance warning of problems and the need for corrective action.
  • Promote compliance, document compliance, prevent blockages and sanitary sewer overflows.
  • Opportunity to minimize manual inspections, reduce frequency of sampling, and reduce cost of maintaining sewer line.
  • Reduce the cost of data acquisition and improve data accuracy.
  • Existing staff to do more with less and should preclude the need to add personnel.
  • Improved maintenance extends the life of equipment and reduces the life cycle cost.

The application of intelligent separation tank monitoring systems offers the greatest return on investment in the following areas.

  • There is high economic growth, new construction.
  • Region is environmentally sensitive.
  • Grease is the number-one cause of blockages and sanitary sewer overflows.
  • Municipality seeks to minimize manual inspections.
  • Municipalities have aggressive FOG program to control discharge of grease intomunicipal wastewater collection systems at the source.
  • Overflows and downtime are unacceptable.
  • Monitoring solution provides first line of defense for expensive on-site wastewater systems.

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