Ozonation of endogenous residue and active biomass from a synthetic activated sludge.

Coupling the activated sludge and the ozonation processes is an efficient, although expensive, solution for sludge reduction. A better knowledge of the mechanisms involved in the degradation of various sludge fractions by ozone is needed to optimize the coupled process. The objectives of this study were to determine the biodegradability of ozone-solubilized endogenous residue, the action of ozone on the active biomass and the solubilization yield of these two main sludge fractions. Batch tests were conducted with slug input of ozone stock solution into fresh or aerobically digested synthetic sludge. Biodegradability of the solubilized endogenous residue was increased by ozonation by up to 0.27 g BOD5/g CODi. Ozone caused biomass lysis, as opposed to an increase in maintenance needs, as shown by a correlation between the decrease in microbial activity and viability. Lysis caused by ozonation was associated with a solubilization of 20% of the lyzed cell COD mass. Solubilization yields were of 9.6 and of 1.9 to 3.6 g COD/g O3 for fresh and aerobically digested sludge, respectively. Design of sludge ozonation processes should account for the variability between the solubilization yield and biodegradability of the various sludge fractions.

Inhibition of biological phosphorus removal in a sequencing moving bed biofilm reactor in seawater.

A new process was developed to achieve denitrifying biological phosphorus removal in wastewaters containing high levels of nitrate and phosphate with a low level of organic matter. This could particularly be useful in recirculating systems such as aquariums or fish farms to prevent accumulation of nitrate and phosphates and to avoid regular cost extensive and polluting water replacement. Phosphorus (P) was removed from the influent in a sequencing moving bed biofilm reactor, stored in the attached biomass and then cyclically removed from the biomass by filling the reactor with anaerobic water from a stock tank. Phosphate was accumulated in the stock tank which allowed for use as fertilizer. The feasibility of the experimental design was demonstrated by using the activated sludge model No. 3 (ASM3) complemented by the EAWAG Bio-P module implemented in the WEST simulation software. A pilot scale experiment was conducted in two identical reactors in two runs: one to treat water from a marine mesocosm, the other to treat a synthetic freshwater influent. No biological phosphorus removal was achieved during the seawater run. During the freshwater run, average P removal efficiency was 20%, of which 80% was attributed to biological removal and 20% to chemical precipitation. The absence of efficiency in seawater was attributed to the high concentration of calcium.

Treatment of freshwater fish farm effluent using constructed wetlands: the role of plants and substrate.

Freshwater fish farm effluents have low nutrient concentrations but high flow rates, resulting in pollutant load, especially phosphorus (P), causing eutrophication. The feasibility was tested of a treatment combining, within a single constructed wetland, the contribution of macrophytes for reducing organic matter and nitrogen (N), with the high efficiency of steel slag and limestone for P removal. Twenty subsurface flow (SSF) basins of 280 L with different combinations of plants (Phragmites communis or Typha latifolia) and substrates (steel slag, limestone, gravel, peat) were fed with a reconstituted fish farm effluent in a greenhouse experiment. Pollutant removal was generally very good under all treatments. N and organic matter removal were correlated with plant biomass while P removal was better in substrates with steel slag and limestone. However, the high pH of the P-adsorbing substrate was detrimental to plant growth so that no combination of plants and substrates could maximise in one step the simultaneous removal of all evaluated pollutants. Therefore, the use of two sequential units is recommended, a first one consisting of a macrophyte planted basin using a neutral substrate to remove organic matter and N, followed by a second unplanted basin containing only a P-adsorbing substrate.