An online monitor for wastewater toxicity and treatability can head off upsets in the microbial life of a wastewater treatment plant. Many plants have systems to detect changes in the dissolved oxygen content of the mixed liquor to determine how the plant is operating. However, once a toxic or inhibitory substance has reached the biological system, not much can be done to reduce the effect on the biomass and prevent its pass-through and discharge.
Online biomonitors using microbial inhibition screening can detect problem influents in time to prevent an upset of the biological process. One such monitor, the Bioscan from N-CON Systems, uses a dissolved oxygen sensor in combination with a submerged biological filter composed of the same microorganisms as the treatment system.
The submerged biological filter and its microbial population are key to the Bioscan design. The filter maintains the layer of microorganisms at a relatively constant thickness through the use of alternating fixed and moving disks that tend to sheer excess growth from the surface. This excess biomass is then flushed from the filter.
The wastewater influent or industrial waste is pumped from a sample overflow chamber 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 an aerator to mix the sample and nutrients, as well as to increase the dissolved oxygen level to saturation. Adjusting the rate of the individual feed pumps will set the ratio of wastewater to biodegradable substrate. The usual ratio is approximately 1:10. 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 can inhibit or kill the microbial 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 DO level is monitored by the systems data recorder through a dissolved oxygen sensor.
The high speed of the biological filters reaction to inhibitory or toxic conditions, combined with the user selected alarm points, gives operating personnel time to take action to prevent an upset. In laboratory testing, response times ranged from eight to 20 minutes with varying concentrations of sodium hypochlorite.
The system will regenerate in as little as 15 minutes, depending on the concentration of the toxic substance.
The monitor has a 4-20 mA DC signal for remote recording and control functions. An alarm contact is provided for early warning of inhibitory or toxic conditions. It can also start automatic samplers and/or other emergency procedures.
To assure the validity of the data, the DO sensing system has an automatic calibration control. At any operator-set time, the control automatically drains the flow cell and air calibrates the DO electrode. The alarm function is locked out during calibration and for approximately five minutes after calibration while the DO system reestablishes equilibrium. Calibration results are stored in the system memory.
Materials for the system cost up to $20 per day. Reagents commonly are available for on-site preparation or in premeasured vials.
