Control of calcium carbonate precipitation in anaerobic reactors

Anaerobic treatment of waste waters with a high calcium content may lead to excessive precipitation of calcium carbonate. So far, no proper methods were available to predict or reduce the extent of precipitation in an anaerobic treatment system. Moreover, it also was not clear to what extent precipitation in an anaerobic reactor can be tolerated because adequate knowledge on the structure and quality of methanogenic sludges with high calcium carbonate content was lacking. In this thesis, the feasibility of anaerobic treatment of waste waters with a high calcium content was investigated.The development of high ash content anaerobic sludges was studied under varying conditions of biomass yield, precipitating quantities of CaCO 3 and particle size of the seed sludge. The location of CaCO 3 precipitation, within the aggregates or in the bulk solution, appeared to be an important factor determining the quality of high ash content sludges. Precipitation in the bulk solution is considered as favourable for the development of these sludges and can in example be stimulated by seeding with the smallest possible granules or by maintaining the phosphate concentration as low as possible. Operating a reactor containing heavy sludge is often problematical because the crystals will tend to agglomerate, resulting in cementation of the sludge bed. This cementation can be retarded by decreasing the degree of pre-acidification.Two different options can be distinguished to reduce the extent of calcium carbonate precipitation in an anaerobic reactor. It was found that phosphate, in concentrations of 0.5 - 5 mgP.l -1, can drastically reduce the quantity of calcium carbonate precipitation. Additives that only inhibit the formation of calcite and not that of aragonite (e.g. iron) are not suitable, because in that case aragonite, of which the solubility is only slightly lower than of calcite, will accumulate in the reactor. The second option is to remove calcium from the influent in a crystallization reactor reusing the anaerobic effluent alkalinity.The amount of calcium carbonate precipitation in an anaerobic reactor can be estimated with equilibrium models as developed in this dissertation, which take into account kinetic factors that influence the apparent solubility of calcium carbonate, being the influent composition, the hydraulic retention time and the value of the crystallization rate constant.In this dissertation, insights have been provided for the design and application of integrated anaerobic-physical-chemical technologies which enable the control of the calcium carbonate precipitation in the anaerobic treatment of waste water with a high calcium content, which appear e.g. in the waste-paper and food processing industries.