Minimization of biomass production in biological wastewater treatment processes
Prof. Ing. Pavel Jeníček, CSc.
In this thesis, the opportunities of minimization of biomass production for biological wastewater treatment processes are presented. Minimization strategies are reviewed from reduction of sludge production during activated sludge process, through anaerobic sludge digestion, to sludge utilization and disposal.
The biomass production can be influenced at the beginning of the wastewater treatment process. Microbial metabolism liberates a portion of the carbon from organic substrates in respiration and assimilates a portion into biomass. To reduce the production of biomass, wastewater processes must be engineered such that substrate is diverted from assimilation for biosynthesis to fuel exothermic, non-growth activities. Such strategies are based on the following mechanisms: uncoupling metabolism (chemical inhibitors, high ratio of the initial substrate concentration to the initial biomass concentration, minimal media, unfavorable temperatures, oxic-settling-anaerobic process), maintenance metabolism (long sludge retention time, membrane bioreactors), lysis and cryptic growth (cell disintegration, biomass ozonation, anaerobic pre-treatment) and predation on bacteria.
Anaerobic digestion technologies have been traditionally employed to reduce volume of the sludge produced during wastewater treatment. During this process, the overwhelming majority of organic matter from the stabilized material is transformed into biogas by the anaerobic microorganisms. Anaerobic sludge stabilization can be further intensified. The intensification methods, discussed in the experimental part of the thesis, are based on the improvement of the biodegradability of particular substrate by means of sludge disintegration and thermophilic anaerobic digestion. The sludge disintegration improves accessibility of the substrate for microorganisms, i.e. improves biodegradability of particular substrate, and releases cell lysate (content of the bacteria cells). Lysate contains some enzymes and growth cofactors stimulating anaerobic degradation process. A partial destruction of cells occurs during the thickening of the excess activated sludge by a special adapted thickening centrifuge. The presence of aerobic cell lysate in the thickened activated sludge caused substantial increment in the methane yield and the biodegradability of thickened activated sludge by 10.9 % in comparison with untreated excess sludge.
The rapid thermal conditioning of digested sludge produced a material with an active anaerobic lysate. The stimulation effect of the anaerobic lysate caused an increment of the anaerobic degradation process. The enhancement of the methane yield of raw sludge was as high as 75 % depending on the lysate amount, i.e. on the amount of the anaerobic sludge treated by rapid thermal reactor.
Lab-scale methods of lysate preparation were also tested; they are: mechanical cell disintegration by mixing, microwave treatment of biomass, ultrasound treatment, thermal high-pressure treatment, repeated freezing and de-freezing. Lab-lysate intensified anaerobic sludge treatment. The proved advantages of the stimulation method with cell lysate are the following: the improvement of the anaerobic biodegradability of organic materials treated, the acceleration of the degradation process, the increment of the methane production and the minimization of digested sludge amount.
Another possibility to intensify the anaerobic degradation of the organic material is increase of the operating temperature in the digesters, when mesophilic temperature is changed to thermophilic. The methanogenic activity of the thermophilic and mesophilic digested sludge was examined during the adaptation of full-scale digesters to an increasing temperature from 38°C to 55°C and the results are summarised in this thesis. The results proved a higher activity of the thermophilic sludge after the addition of glucose, acetate and propionate in comparison with the mesophilic sludge; an acceleration of biochemical reactions, a higher efficiency in the degradation of organic matter and destroying pathogens, a higher methane production rates and the methane yields of the primary sludge, activated sludge and raw sludge under thermophilic conditions. The stability of the degradation process at an increased substrate load was also better. The higher efficiency of the anaerobic digestion process results in minimization of the stabilized sludge amount and its higher quality. A thermophilic process consequence was a sludge water composition shift, the concentrations of the dissolved COD, N-NH4+ and P-PO43- slightly increased.
Treatment, utilization and disposal of sewage sludge from wastewater treatment plants are also discussed in the presented thesis. The mentioned alternatives for sludge disposal are the use for agricultural purposes (land application of the digested sludge, composting), utilization of sludge as a sorption material, thermal treatment (incineration, pyrolysis, high-pressure wet incineration, plasma oxidation, solidification and conversion of sludge into fuel and construction material) and sludge dumping at landfill.