INFLUENCES OF WASTE-WATER COMPOSITION AND OPERATING-CONDITIONS ON ACTIVATED-SLUDGE BULKING AND SCUM FORMATION

Bulking and scum formation (scumming) are common phenomena hi activate d sludge plants. Four main problem groups with filamentous microorgani sms with different growth requirements are distinguished: (1) ''Aerobi c Bulking'', (2) ''Scumming due to Actinomycetes'', (3) ''Low F/M Bulk ing and Scumming'' and (4) ''Bulking due to Sulphide Oxidising Bacteri a''. Various wastewater fractions, such as readily biodegradable subst rate, surfactants, hydrophilic and lipophilic slowly-biodegradable sub strate, inoculated filamentous microorganisms and sulphide, strongly i nfluence the biocenosis composition. Apart from wastewater composition , operating conditions, such as solids retention time, reactor configu ration, mechanical stress by recirculation pumps and insufficient oxyg en supply, are often decisive for die occurrence of a bulking or scumm ing problem, as well A predictive scheme for the possible occurrence o f the four main problem groups is presented for different combinations of wastewater compositions, operating conditions and reactor design. AII four problem groups can occur in aerobic nitrifying systems. In no n-nitrifying systems, only ''Aerobic Bulking'' and ''Bulking due to Su lphide Oxidising Bacteria'' are expected, depending on wastewater comp osition and operating conditions. in systems with a large anoxic mass fraction, ''Low F/M Bulking and Scumming'' is typically the only probl em group. Since wastewater fractions can hardly be manipulated in prac tice, at least operating conditions and reactor design should be optim ised in order to obtain a better performance of the plants.