Effect of Chemical Pretreatment on the Aerobic, Thermophilic Digestion of Piggery Wastes

Abstract

ABSTRACT LABORATORY scale vessels were used to determine the effects of chemical pretreatment on the aerobic, thermophilic digestion of dilute piggery waste (= 22 TS g/L1). The wastes were untreated or mixed with either sodium hydroxide (NaOH 10 percent W/W), ammonium chloride (NH4C1 10 percent W/W) or sulphuric acid (H2S04 5 mL 1 M/100 g dry matter). Treated wastes were stored at ambient temperature for up to 2 days prior to digestion at 55 C for an average retention time of 2 days. Aerobic, thermophilic digestion alone or in combination with chemical pretreatment resulted in significant (P < 0.05) reductions in the levels of volatile solids (VS) and biochemical oxygen demand (BOD5). Although NaOH pretreatment increased the mean BOD5 of the influent by 34 percent the BOD5's of the digested effluents were not significantly different. The addition of alkali resulted in significantly (P < 0.05) lower levels of VS compared with the untreated effluent whereas H2S04 pretreatment resulted in less effective VS degradation than in the control digestion. NH4C1 pretreatment did not have a significant effect on the removal of organic pollutants. The neutral detergent fibre fraction in the wastes was reduced from 55 percent dry matter to approximately 31 percent of dry matter as a result of NaOH pretreatment; the level after digestion was 24 percent. Aerobic, thermophilic digestion of untreated wastes reduced the levels of fecal coliforms (FC) from a mean of 2.5 x 106 organisms/mL to 2.4 x lOVmL. NaOH pretreatment enhanced bacterial removal by reducing the numbers in the influent to 3.3 x lOVmL while maintaining the same reduction (< lOVmL) during thermophilic digestion. The other treatments had no effect on FC numbers in the influent and resulted in an increased number of organisms in the effluent.