Inhibition Monitoring for Rapid Wastewater Screening

By Steven Wooten

Simplified, rapid methods for assessing the inhibition potential of wastewater are essential for controlling discharges from industrial waste generators to wastewater treatment plants. One method for identifying shock loads uses a biomonitoring instrument that combines the action of living microorganisms, representative of the treatment system, with a method of determining and recording the impact of a continuous waste stream on their respiration.

The Bioscan system from N-Con Systems combines the power of modern computers and a dissolved oxygen sensor with a submerged biological filter composed of the same microorganisms as in the wastewater treatment system. When installed in a treatment plant, it can be used for continuous, real time monitoring of inhibitory or toxic materials in the influent.

Strong industrial waste can disrupt the normal biological activities of an activated sludge process. Such shock loads can result in the treatment system failing to function properly and can lead to the discharge of untreated inhibitory and toxic material to the receiving waters.

Frequent analysis of wastewater to determine its composition is both costly and time consuming. Delay in data availability makes traditional tests such as 5 day Biochemical Oxygen Demand (BOD5) and Whole Effluent Toxicity tests unsuitable for process control. In addition, these tests do not necessarily indicate the inhibition potential to the activated sludge.

Operating Principle

The operating principle of the Bioscan system is based on the respiration of microorganisms. Aerobic microorganisms that perform the microbial inhibition screening of wastewater require oxygen as they break down organic matter or substrate. If their activity is normal, the dissolved oxygen in the water is consumed. Toxic or inhibitory materials prevent biomass respiration and little or no dissolved oxygen is used. Therefore, the quantity of dissolved oxygen and the rate at which it is consumed is an indication that the material flowing through the biological filter is biodegradable, inhibitory or toxic to the microbial population.

The key to the system's operation is a submerged biological filter that supports a microbial population similar to the wastewater treatment plant's activated sludge process. The upflow filter is designed to maintain the layer of microorganisms at a relatively constant thickness. This is accomplished by alternating fixed and moving disks (which rotate at 30 rpm) that tend to sheer excess growth from the disks' surface. The excess biomass is flushed from the filter to a drain.

Wastewater influent or industrial waste is pumped to a sample overflow chamber in excess. A subsample of the waste is pumped into a aerator and mixed with a readily biodegradable substrate and nutrient stock.

The substrate assures that the biomass will have sufficient food for vigorous oxygen demand. Air is introduced in the aerator to mix the sample and nutrients, as well as to increase the dissolved oxygen level to saturation. The ratio of wastewater to biodegradable substrate can be set by adjusting the rate of the individual feed pumps. The usual ratio is approximately 1 to 10. The mixture flows through the biological filter at a rate of 40 ml per minute. This rate allows the healthy biomass to consume most of the available food and dissolved oxygen.

If the wastewater contains materials that inhibit or kill the microbial inhibition screening culture in the filter, the microorganisms consume little or no oxygen. Thus, the quantity of dissolved oxygen (DO) passing out of the filter is a direct indication of inhibition or toxicity. The dissolved oxygen remaining in the effluent of the filter is monitored by the system's data recorder through a dissolved oxygen sensor. The results are continuously recorded by a computer. Alarms may be set through the computer to provide a local or a remote signal, such as on the central control panel.

Application Data

The high speed of the biological filter's reaction to inhibitory or toxic conditions, combined with the user selected alarm points, gives operating personnel adequate warning so that remedial action may be taken. In laboratory testing, response times ranged from 8 to 20 minutes with varying concentrations of sodium hypochlorite.

During testing with 100 ppm sodium hypochlorite, the Bioscan system reached the alarm point of 1.5 ppm dissolved oxygen within 10 minutes. A feed pump control is set at 3 ppm dissolved oxygen. When the dissolved oxygen exceeds this level, the feed pump is stopped. This protects the biological filter from excessive exposure to highly inhibitory waste streams by pausing the introduction of the offending sample. The nutrient input continues to dilute the material in the biological filter. When the filter's respiration recovers enough to drop the dissolved oxygen level below 2.9 ppm, the feed pump resumes.

Self-testing of the dissolved oxygen system is provided by the computer on a daily basis. The system drains the oxygen probe chamber and performs an air calibration every 24 hours. Sample pump delivery adjustments and dissolved oxygen system calibration can be done manually by plant operators without interfering with the computer settings. Completed daily records are stored in the data recorder's memory for up to four days. The computer automatically assigns the correct date to each chart. After four days the older data is over-written by current data.

Data records can be transferred to a personal computer with the data transfer unit or direct connection with a PC using an RS-232 interface cable. Data can be recalled by the computer for review in both graphic and numeric forms, printed out, or transferred to spreadsheets or reports.

A Bioscan inhibition monitor was tested at the Crooked Creek Water Reclamation Plant in Gwinnett County, Ga., in the fall of 1997. The treatment plant primarily receives residential wastewater and accepts some trucked in wastes from landfill leachates and septic tanks. The monitoring system was installed before the primary clarifiers, sampling screened untreated wastewater. This location afforded the earliest detection site for inhibitory inflows. A carboy of nutrients was mixed each day to maintain a source of easily biodegradable substrate for the biological filter.

During testing from November 7th to November 10th, as expected with domestic wastewater the monitor documented no inhibitory or toxic materials entering the plant. The respiration of the biological filter did show a problem early on November 9th because the carboy of nutrients had not been replaced. A sign-up sheet was started to prevent this from recurring.

Conclusion

Biomonitoring using this technology of combining the representative microbial inhibition screening of wastewater and the power of modern computers will help to reduce the incidence of damage to wastewater treatment systems. These systems offer an affordable, timely alternative to Whole Effluent Toxicity testing and routine batch respirometric tests.

The system is intended for routine screening of wastewater to document the treatability of wastes and to warn of potential problems. This will help to protect receiving waters from discharges of toxic and inhibitory materials and allow for a more proactive approach to wastewater management.