Conventional Primary Sludge Fermentation Configurations for Enhancing Biological N&P Removal

A primary sludge fermenter is a reactor or unit process found on site at a wastewater treatment plant in which complex particulate and soluble substrates present in the primary sludge and wastewater are anaerobically broken down to form short-chain volatile fatty acids (SCVFAs) through acid fermentation. The SCVFAs are used to develop stable populations of phosphate accumulating organisms (PAOs) and denitrifying bacteria in the biological nutrient removal (BNR) process, thus helping the process achieve low total phosphorus and total nitrogen concentrations by biological means. In the past 30 years, primary sludge fermenters have been used in BNR plants in Canada, the USA, Europe, South Africa, Australia and New Zealand. Several facilities having primary sludge fermenters are reported to be unable to meet their effluent phosphorus limits biologically when the fermenters are taken out of service. This paper describes the following five commonly applied primary sludge fermenter configurations together with the advantages and disadvantages of each, and typical control features: 1) activated primary sedimentation tanks; 2) complete mix fermenter; 3) single-stage static fermenter/thickener; 4) 2-stage fermenter/thickener; and 5) unified fermentation and thickening (UFAT) fermenter.The two principal control parameters for the operation of primary sludge fermenters are the fermenter solids retention time (SRT) and hydraulic retention time (HRT). The fermenter SRT is controlled by adjusting the solids inventory and the sludge wastage rate. By increasing the fermenter SRT, the growth of slower growing fermentative organisms is favored, and more complex molecules and higher acids are produced. Conversely, decreasing the SRT favors the growth of faster growing organisms, resulting in simpler biochemical pathways and the production of acetic acid and, and to a lesser extent, propionic acid. The ratio of SCVFA produced per VSS added to a fermenter has a fairly broad range, from 0.05 to 0.3 g-VFA/g-VSS added.