Problems In Testing Efficacy For Disinfection Of Wastewater Treatment Plant Effluent

By: J. RODNEY DICKERSON, P.E.

Presently approved EPA methods for determining effectiveness of disinfection for wastewater treatment plant effluent utilizes "plate counts" which involve taking a sample of the wastewater and using it to inoculate a growth media and thereby cultivate viable bacteria for a period of time. The cultures are then subjected to microscopic examination and the number of "colony forming units" are determined and reported. Success or failure is determined by the quantity of bacteria grown on the plates.

Research at the Center for Biofilm Engineering (CBE) at the University of Montana has proven two very important factors. One, that bacteria bound in biofilm material (exocellular polysaccharide substance or EPS), also known as "sessile," can survive applications of disinfectants that would otherwise kill free-swimming (planktonic) bacteria. In fact, it may take from 100 up to 1,500 times as much biocide to kill sessile bacteria as would otherwise be required for planktonic bacteria. Two, sessile bacteria can survive exposure to biocides and not grow on media as would otherwise be expected.

It is also well known that most bacteria in wastewater treatment processes exist in particles, commonly referred to a "floc." These floc particles are, in fact, sessile compositions of many bacteria. Previously believed formed in the plant, these particles are now believed produced almost exclusively within the biofilm or slime that resides on the interior of the collection system piping. This slime sloughs on a continuous basis, releasing sessile particles. The piping surface area is extremely large, so large quantities of sessile bacteria are being released. Furthermore, research has shown that there are periods of very large slough events whereby massive amounts of sessile particles are released.

Current testing with plate counts takes up to two days to complete. This information is of little use in the control of the treatment process to mitigate release of large quantities of potentially pathogenic bacteria, indeed if such mitigation could be accomplished considering the implications of resistance of sessile state bacteria to disinfection.

Present plate count testing often indicates the presence of large quantities of fecal bacteria in the absence of significant Total Suspended Solids (TSS) and with the appropriate residual levels of disinfectant. Likewise, significant TSS may be reported and fecal bacteria are reported as absent. Often times these variations are accredited to poor sampling or laboratory practices. Similarly, infrequent "spikes" in influent TSS in the absence of material changes in other measured parameters are accredited to poor sampling or laboratory practices.

Inasmuch as human coliform bacteria account for the majority of bacteria in fecal material and subsequently are present in native collection system piping biofilm structure in large numbers, it would follow that such bacteria would be present in the sessile particles and some portion could pass through the wastewater treatment process and exit, surviving disinfection and detection, yet still be viable.

This writer proposes that new technology entering the market that allows for almost immediate detection of viable bacteria, whether planktonic or sessile, be investigated as soon as possible. The continuing threat to our surface waters from contamination by potentially pathogenic bacteria demands urgency. Each spring we see sudden increases in fecal coliform in our coastal waters. Data is now being developed that shows, from early indications, that collection system piping undergoes major sloughing events during this period that would provide a means to produce such events. A first step towards controlling this problem is to detect viable sessile state bacteria exiting the treatment process that are otherwise undetected by present protocols and methodologies.

SOURCE: In-Pipe Technology Company, Inc.