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In order to assess the toxicity of compounds in wastewater and activities in reactors a precise, reproducible, fast and simple batch test is used. This research was focused on the development of such a test to determine denitrifying activity. The test is based on measuring the rate of gas production during denitrification, by tracking both the pressure and the gas composition in the headspace of a sealed bottle. The optimum pH for batch activity was found to equal the pH in the lab-scale reactor (9.5) from which the sludge originated. The minimum COD/N ratio required for linear gas production curves with acetate/nitrate was 3.5 g COD/g N. A significant increase in specific activity at low solids concentrations (<2 g VSS/l) was observed. No clear relation between activity, DNA and RNA content of the sludge after the test and initial substrate to biomass (S0/X0) ratio could be established, indicating that other factors play a role in determining the activity, possibly adsorption and substrate mass transfer. Standardization of batch tests should therefore include the solids concentration.

Free-swimming Annelida occasionally occur in very high densities in WWTPs (WasteWater Treatment Plants) and are nowadays applied for waste sludge reduction, but their growth is uncontrollable. In order to get more insight in the population dynamics of these free-swimming Annelida, and relate their presence to process characteristics, nine ATs (Aeration Tanks) of four Dutch WWTPs were regularly sampled over a 2.5-year period. For each species, peak periods in worm population growth were defined and population doubling times and half-lives calculated. Peak periods and doubling times were compared to those in natural systems. Process characteristics were obtained from the plant operators and related to the worm populations by multivariate analysis for the first time in large-scale WWTPs. The species composition in the WWTPs was limited and the most abundant free-swimming Annelida were in decreasing order Nais spp., Aeolosoma hemprichi, Pristina aequiseta, Aeolosoma variegatum, Chaetogaster diastrophus, and Aeolosoma tenebrarum.This latter species had never been found before in WWTPs. Worm absence sometimes coincided with the presence of anoxic zones, but this was possibly overcome by higher temperatures in the WWTPs. Worms were present all year round, even in winter, but no yearly recurrences of population peaks were observed, probably as a result of stable food supply and temperature, and the lack of predation in the WWTPs. Peak periods were similar between the ATs of each WWTP. The duration of the peak periods was on average 2-3 months for each species and the population doubling times in the peak periods were short (on average 2-6 days), which also corresponds to a stable favorable environment. The disappearance of worm populations from the WWTPs was presumably caused by declining asexual reproduction and subsequent removal with the waste sludge. Multivariate analysis indicated that 36% of the variability in worm populations was due to spatial and temporal patterns only. In addition, no more than 4% of the variability in worm populations was related to variations in process characteristics only and worm presence was usually associated with better sludge settleability. In conclusion, our data from large-scale Hydrobiologia (2008) 605:131-142 DOI 10.1007 WWTPs suggest that growth of free-swimming Annelida still seems uncontrollable and that their effects on treatment processes are unclear, which makes stable application in wastewater treatment for sludge reduction difficult.

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