Anaerobic granular sludge : characterization, and factors affecting its functioning

Many UASB reactors are designed in such a fashion that the presence of granular sludge is necessary for a proper purification process. For achieving an optimum wastewater purification with such reactors, knowledge of the factors that determine the growth, retention and disintegration of anaerobic granular sludge is essential. The present research focused on gaining more insight in the factors determining the growth and quality of anaerobic granular sludge. For determining the total available pore volume and the pore diameter distribution of granular sludge, a method based on size exclusion chromatography has been developed. For most types of sludge, the available pore volume varies between 40% and 80%, granules with a lower porosity probably contain layers that are impermeable to substrate. Small granules were found to have a considerably higher porosity and a higher maximum methanogenic activity than larger granules from the same sludge sample. In applying higher sludge loads, the average granule diameter increases. A decrease in sludge load or changes in substrate composition result in a weakening of sludge granules, probably due to a lack of substrate for the bacteria in the centre of the granules. In view of the stability of methanogenic granular sludge, this should be attributed to the dying off of the acidifying population inside the sludge granules. The process in UASB reactors is strongly influenced by the composition and degree of pre- acidification of the wastewater. It was found that non-acidified gelatine can be treated in an one- phase UASB reactor without difficulties up to a sludge load of 1.2 gCOD.(gVSS.d) -1. However, sucrose-containing wastewater can be treated only at a sludge load below 0.5 gCOD.(gVSS.d) -1. At higher sludge loading rates nonacidified sucrose in the reactor influent can cause problems with regard to sludge retention. However, too much pre-acidification of wastewater can also cause problems. Acidogenic bacteria suspended in the influent may cause very serious flotation of granular sludge. In treating sulphate-containing wastewater, sulphate-reducing bacteria (SRB) and methane- producing bacteria (MPB) will compete for substrate. It is often assumed that MPB can maintain in high-rate anaerobic reactors because of the poor ability of SRB to attach themselves compared to MPB. However, the present study shows that sulphate-reducing bacteria are capable to maintain in granular sludge. They were even found to be able to attach themselves: on pumice stone as carrier material, purely sulphidogenic aggregates were formed. A negative effect of a deficiency in phosphate on the methanogenic activity of granular sludge was found to be fully reversible in the presence of phosphate. The phosphorus content of granular sludge from the laboratory reactors fed with gelatine varied between 6 mgP.gVSS -1for sludge from reactors fed with influent that contained almost no phosphate and 10.5 mgP.gVSS -1for sludge from reactors with a sufficient supply of phosphate. Deficiency in phosphate was found to be easily demonstrable: an increase in methanogenic activity after phosphate dosage and/or a rapid uptake of phosphate are clear indications of a deficiency in phosphate in granular sludge.