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Chemically enhanced primary treatment of municipal wastewater has gained acceptance in Mexico and other countries There is evidence that enhanced primary treatment sludge (EPTS), with aluminum sulfate Al2(SO4)3 as coagulant, inhibits anaerobic digestion. In this paper, results obtained by the UNAM research group, in order to assess and overcome this limitation, are presented. The combined and individual effects of aluminum and sulfate on the anaerobic digestion of sludge from enhanced primary treatment were evaluated in 1 L capacity semi continuous reactors. It was found that aluminum inhibits the specific methanogenic activity of methanogenic and acetogenic bacteria resulting in a 50% to 72% decrease. Sulfate also inhibits (48% to 65%) the activity of the same type of bacteria. However, the combination of aluminum and sulfate resulted in a higher inhibition, especially of the hydrogenophilic methanogenic bacteria. Acidification of anaerobic reactors fed with EPT sludge is due, among other causes, to the concurrent presence of aluminum and sulfate. Two additives (lyophilized bacilli) and a solution of micronutrients (Fe, Co, Ni and Mo) were tried separately and in combination in batch test runs, in order to assess their effect on the anaerobic digestion of waste sludge from an enhanced primary treatment (EPT) of municipal wastewater. The anaerobic thermophilic digestion of sludge from aluminium sulfate EPT will be improved with the addition of Fe: 4.5 mg/g VS, Ni: 0.75 mg/g VS, Co: 0.45 mg/g VS, Mo: 0.09 mg/g VS and bacilli additive: 12 mg/g VS.

Two thermophilic anaerobic digesters were fed with EPT sludge in order to confirm the results obtained in the batch experiment. Digester T2 showed a better behaviour if compared with control digester T1, at organic loads of 1.2 and 1.6 kg TS/m3.d. The application of bacilli and micronutrients additives had positive effects in the anaerobic digestion of enhanced primary treatment sludge, since methanogenic activity was improved, volatile fatty acids were lowered, volatile solids removal was increased and a more balanced system was established, if compared with the control digester T1.

Thermophilic anaerobic digestion is a suitable stabilization process for production of Class A biosolid, according to the Mexican standard NOM-004-SEMARNAT-2002.
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Document Type: Research Article

Publication date: 01 January 2005

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