In recent years, increased level of awareness has prompted the government to come up with a more stringent control measures and setting higher effluent discharge standards to protect the general public from the effects of toxicity. Detailed studies have been carried out and wastewater coming from the industrial sectors has been identified to be major contributor resulting in the presence of toxic compounds in the sewer system and what causes a high level of concern is that these pollutants can still be found present in the effluent discharge even after the wastewater has undergone conventional treatment processes. Most of these chemicals are believed to be from textile, latex, foundry and pharmaceutical industries whereby use of active compounds are known to carry high concentration of heavy metals and synthetic pollutants. Nowadays even the domestic wastewater discharge seems to carry an almost equivalent amount of these toxic compound particularly those that originate from consumer cleaning products.
Toxicity evaluations have so far largely focused on effluent analysis rather than influent samples with large number of tests and unit of measurements developed to study the effects involving the use of bioassays with different species of organisms as test subjects. Basically what is determined is measuring the duration of exposure on whether it can result in acute or chronic condition and also the effect it can cause towards the mortality, reproduction and motility of the test subjects. These have been discussed before and elaborated in details in another chapter related to effluent toxicity testing. For influent evaluation however, the approach is more towards evaluating the effects of the toxic compounds towards the wastewater treatment plant processes and its treatment capability. Indirectly, efficiency also comes into picture here as there is also another area of concern as intoxicated biological sludge system will also suffer from reduced capacity and this can also lead to the wastewater treatment plant unable to cope with the inflow of influent and thus leads to discharge with lower quality. Since most conventional systems rely mainly on biological pathways in order to breakdown organic compounds and convert most toxic material into less harmful by products, presence of these toxic substances could render the whole biological system ineffective.
Toxic shocks are the most feared and problematic scenario affecting the management of the treatment system. It is usually caused by sudden introduction of non-native, new toxic compounds not commonly fed to the microorganisms or through a sudden increase in excessive concentration of these compounds. It is also defined as the condition whereby the microorganisms will find it unable to adjust to the new environment and thus lead to reduced activity resulting in lower treatment capacity and efficiency. This condition can be as a result of either inhibition or inactivation of certain microorganisms that perform the biological degradation or in the worst case scenario the death of the whole stock of bacteria. Decreased treatment capacity can also lead to deflocculation which can cause sludge washout whereby it is a condition defined as break-up of flocs into smaller fragments which has lower gravity settling property. This results in carry over of suspended solids into the final discharge affecting the overall quality and since it cannot perform at its peak performance, whatever toxic compound originally present will still be left untouched. Once toxic shock happens, the treatment system can sometimes that weeks up to months before it can fully recover to its former condition and restoring the whole system back to its previous treatment capability can be extremely costly, since it also involves disposing large volumes of intoxicated dead sludge, and re-seeding the system with new stock of microorganisms through bioaugmentation.
Thus, laboratory-scale test facilities are usually setup in large treatment plants in order to find out the outcome on the presence of certain toxic chemical coming from the influent. This will help the management teams to be more prepared and create an early warning system and having a better crisis management program to avoid the total breakdown of the whole system. Some of the most common tests carried out on influent toxicity detection has been elaborated in details in Love and Bott (Love and Bott, 2000) and an updated version with new information is also available in Ren (Ren, 2004) discussing the advantages and weakness of some of the methods. Basically these are 3 common approaches used for the influent testing which are 1) Use of Bio luminescent Bacteria 2) Nitrification Inhibition and 3) Respirometry.
Use of Bio luminescent Bacteria
Similar as in the approach of effluent testing using bioassays, this test method involves the use of deep sea marine bacterium (Vibrio fischeri) as the test subject and then studying the effects of introduction of toxic compounds. Since the bio luminescence emits light during biochemical reaction, this will help to track the microbial count and make it easier to monitor the bacterial activity. However there are strong arguments with regards to the accuracy of the test method mainly because it is using test subjects which are not directly related to the common bacteria present in the activated sludge system. More over, this type of bacteria is a marine microorganisms which raises more doubts and questions about its accuracy.
Nitrification inhibition test involves studying the effects of introduction of toxic substances towards the nitrification process. Since nitrification is performed largely by two groups of bacteria (Nitrosomonas and Nitrobacter) in a two-stage conversion process, from ammonia to nitrite and then to nitrate, the test method can focus either on the first step or the whole complete process taken into account. The reason on why nitrification is selected is due to the fact that that the whole chemical reaction is very sensitive to presence of toxic substances and thus provides a closer-to-actual representation. The effects can be measured either by oxygen consumption, rate of reduction in ammonia or the efficiency of conversion from nitrite or nitrate. However, points of contention regarding the accuracy and viability of the test method is that most biological treatment plants consist of non-nitrifying and heterotrophic bacteria with nitrifying bacteria represents only a small portion of the general population.
In Respirometry, a test reactor is set up so that the specific oxygen uptake rate (SOUR) of activated sludge can be monitored with the introduction of the toxic compound. Since SOUR is directly related to the metabolism and biological activity of the microorganisms, it can be used to represent the effects of toxic substances present in the system and how it could affect the growth and reproduction of the bacteria.