07%). The results of the present study correspond to the findings of previous investigators who also reported an increase on COD when working on the removal of nutrients [27] or on the tolerance of Ni2+/V5+[21, 22] by the same test protozoan species in activated sludge mixed liquor. As opposed to this, Pala and Sponza [56] reported an efficient removal of COD in activated sludge with the addition of Pseudomonas sp. Musa and Ahmad [57] also

reported a reduction on COD of up to 94% in click here wastewater when using Small molecule library some industrial wastewater bacterial isolates. Statistical evidence indicated strong and moderate positive correlations consecutively between growth performance and some heavy metal removal regardless of pH, COD increase and DO removal, which could be attributed to combined microbial activities such as the biosorption EVP4593 in vitro of metals to cell surfaces [58], release of extracellular polymeric substances during the detoxifying process of heavy metals as well as die-off of microbial cells [59]. The weak correlations between protozoan counts and other parameters could also be attributed to the inhibition of the protozoan isolates throughout

the experimental study [43]. It is well known that the pH is also an important and limiting parameter in wastewater treatment systems for the growth and activity of several organisms. In bioremediation processes, acid-tolerant microorganisms are viewed as being beneficial for the treatment of highly polluted wastewater from the mines or industry [57, 60]. However, by investigating the variations of pH in the polluted industrial wastewaters, which initially had a pH of approximately 4, a slight fluctuation of pH in the inoculated industrial wastewaters was observed throughout the study period (Tables  2). Although the range of pH values for several biological activities is very narrow and ranged between 6 and 9 [48], this finding revealed that all test isolates except Aspidisca sp. were able to grow

in an aqueous solution with a pH value of approximately 4. Akpor et al. [27], however, reported an increase in the pH value in activated sludge inoculated with some selected wastewater protozoan isolates. Conclusions The outcomes of the study revealed that the South African industrial wastewater samples were highly polluted with various heavy metals, which resulted in growth inhibition NADPH-cytochrome-c2 reductase of test isolates, especially protozoa. However, the growth of Pseudomonas putida, Bacillus licheniformis and Peranema sp. were not considerably affected by the toxic effect of the metals in the culture media. The efficiency of bacteria and protozoa in removing heavy metals from the polluted industrial wastewater mixed-liquor were found to be significantly different (p < 0.05) for most of the heavy metals with the exception of Cd, Zn, Cu, Pb and Al. In general, bacterial isolates exhibited the highest removal rates of most of the heavy metals compared to the protozoan isolates.