Subsequent Anaerobic Digestion Research Articles

Effect of autothermal treatment on anaerobic digestion in the dual digestion process

The performance of two laboratory mesophilic anaerobic digesters was studied to determine if the claims for improved mesophilic anaerobic digestion following autothermal pre-treatment are valid and if the benefits claimed are affected by the mesophilic digester SRT. One digester was fed sludge taken after a full-scale autothermal reactor and the other was fed sludge from the same plant just before the autothermal treatment unit. Autothermal treatment did not significantly increase volatile solids destruction kinetics during subsequent anaerobic digestion. The VS destruction rate was 2.7 to 2.8% per day with or without autothermal treatment. Autothermal pre-treatment did not affect methane production per unit VS destroyed at anaerobic digester SRTs ranging from 6 to 12 days. In the digester fed autothermal sludge, higher alkalinity and lower VFA concentration produced a lower VFA to alkalinity ratio, indicating the autothermal pretreatment has the potential to make anaerobic digester operation more stable compared to anaerobic digestion only. Autothermal pretreatment consistently reduced fecal coliform to below detection limits and fecal coliform remained below detection limits during anaerobic digestion.

A mechanical pretreatment of waste activated sludge for improvement of anaerobic digestion system

This study evaluated the effectiveness of sewage pretreatment as it affected the subsequent anaerobic digestion of waste activated sludge (WAS). The key to this sludge pretreatment process was that microorganism cells within WAS were ruptured by mechanical jet and smash under pressurized conditions, 5–50 bar pressure. Cell rupture was estimated by soluble protein concentration (SPC) because the cytoplasm of microorganisms is mainly composed of protein. The SPC in the WAS varied significantly before and after pretreatment. In particular, cell rupture increased from 6% to 43% with the increase of the pretreated pressure (5–30bar), volatile solids concentration and thickening time of WAS. In the WAS pretreatment with long-thickening time (12h < thickening time ≤ 48h), cell rupture was monitored 5–10 % higher than short-thickening time (Oh <thickening time ≤ 12h). Generally, it was observed that higher anaerobic digestion efficiencies of WAS could be obtained according to the increase of microorganism cell rupture through mechanical pretreatment of WAS.

Anaerobic digestion of primary sludge from chemical pre-precipitation

Many existing wastewater treatment plants are to be upgraded for phosphorus removal. In our study, ferric chloride was used as a coagulant in pre-precipitation of municipal sewage. Using a Fe/P molar ratio of 0.8, removal efficiencies for suspended solids (64%), chemical oxygen demand (50%), Kjeldahl nitrogen (22%), total phosphorus (43%) and orthophosphate (51%) were obtained. Anaerobic digestion of raw primary sludge yielded a volatile solids (VS) destruction of 35% at VS loading rates of 0.60 to 0.79 kg VS/m3.d. Digestion of chemically enriched primary sludge (CEP-sludge) yielded a VS destruction of 57% at a VS loading rate of 1.36 kgVS/m3.d. Comparison of the methane production per kg VS destroyed (519 to 612 1 CH4/kgVS destroyed for primary sludge, 299 to 395 1 CH4/kgVS destroyed for CEP-sludge) evidenced a change in the composition of the organic material after precipitation with coagulants; the latter sludge was enriched in less reduced compounds. The precipitated phosphorus was not released to the supernatant during anaerobic digestion. No evidence for reduced digester stability was found for digestion of CEP-sludge. These results indicate that retro-fitting a plant by chemical pre-precipitation and subsequent anaerobic digestion of the CEP-sludge is a feasible option.

A mechanical pretreatment of waste activated sludge for improvement of anaerobic digestion system

This study evaluated the effectiveness of sewage pretreatment as it affected the subsequent anaerobic digestion of waste activated sludge (WAS). The key to this sludge pretreatment process was that microorganism cells within WAS were ruptured by mechanical jet and smash under pressurized conditions, 5–50 bar pressure. Cell rupture was estimated by soluble protein concentration (SPC) because the cytoplasm of microorganisms is mainly composed of protein. The SPC in the WAS varied significantly before and after pretreatment. In particular, cell rupture increased from 6% to 43% with the increase of the pretreated pressure (5–30bar), volatile solids concentration and thickening time of WAS. In the WAS pretreatment with long-thickening time (12h < thickening time ≤ 48h), cell rupture was monitored 5–10 % higher than short-thickening time (Oh <thickening time ≤ 12h). Generally, it was observed that higher anaerobic digestion efficiencies of WAS could be obtained according to the increase of microorganism cell rupture through mechanical pretreatment of WAS.

Anaerobic digestion of primary sludge from chemical pre-precipitation

Many existing wastewater treatment plants are to be upgraded for phosphorus removal. In our study, ferric chloride was used as a coagulant in pre-precipitation of municipal sewage. Using a FeIP molar ratio of 0.8, removal efficiencies for suspended solids (64%), chemical oxygen demand (50%), Kjeldahl nitrogen (22%), total phosphorus (43%) and orthophosphate (51 %) were obtained. Anaerobic digestion of raw primary sludge yielded a volatile solids (VS) destruction of 35% at VS loading rates of 0.60 to 0.79 kg VS/m3.d. Digestion of chemically enriched primary sludge (CEP-sludge) yielded a VS destruction of 57% at a VS loading rate of 1.36 kgVS/m3.d. Comparison of the methane production per kg VS destroyed (519 to 612 1 CH4/kgVS destroyed for primary sludge, 299 to 395 1 CH4/kgVS destroyed for CEP-sludge) evidenced a change in the composition of the organic material after precipitation with coagulants; the latter sludge was enriched in less reduced compounds. The precipitated phosphorus was not released to the supernatant during anaerobic digestion. No evidence for reduced digester stability was found for digestion of CEP-sludge. These results indicate that retro-fitting a plant by chemical pre-precipitation and subsequent anaerobic digestion of the CEP-sludge is a feasible option.

Aerobic thermophilic pretreatment of mixed sludge for pathogen reduction and Nocardia control

Pilot scale experiments have been conducted to investigate the use of aerobic thermophilic pretreatment (ATP) of mixed (waste activated and primary) sludge before mesophilic anaerobic digestion to meet Class A sludge pathogen control requirements. The ability of ATP to control Nocardia present in the waste activated sludge, and hence prevent foaming in the subsequent anaerobic digestion, was also investigated. Aerobic thermophilic pretreatment of mixed sludge before anaerobic digestion was able to produce sludge that can meet the following Class A pathogen reduction requirements: fecal coliform density &lt;1 000 MPN/g total solids or Salmonella sp. density &lt;3 MPN/4 g total solids; time and temperature requirements (1.8 hr at 63°C); enteric virus density &lt;1 PFU/4 g total solids; and vector attraction requirement (&gt;38% volatile solids reduction). ATP was able to control Nocardia to below detectable levels, whereas the Nocardia levels in the control anaerobic digester were ≥105 intersections/g VSS when both were fed mixed sludge containing Nocardia filament counts in excess of 105 intersections/g VSS. The ATP system was also able to produce higher volatile solids (VS) reduction, higher methane content in sludge gas, and lower sludge supernatant COD when compared with those achieved in the control anaerobic digester.

Effects of Thermal Pretreatment of Sewage Sludge on Anaerobic Digestion

The study examines the constraints on thermal pretreatment of sludge at temperatures in excess of 100 °C and its effects on subsequent anaerobic digestion. The table below summarizes the maximum measured increase rates and the corresponding optimum pretreatment temperatures as a function of the type of sludge tested. The influence of the sludge type is considerable.Sludge TypeGas Yield without/with PretreatmentMaximum increase RateOptimum Pretreatment Temperaturel/kg VSinfl%°CDigested Sludge60.4223.0+ 269.2180Excess Sludge SLR = 0.03124.7278.1+ 123.0135Excess Sludge SLR = 0.15314.7546.7+ 73.7170Excess Sludge SLR = 2.00234.7334.2+ 42.4135Primary Sludge259.0326.6+ 20.3170Excess Sludge SLR = 0.60596.8600.0+ 0.5135 Calculation of detailed energy balances for various sludge treatment systems with and without thermal pretreatment revealed a clear economic advantage for thermal pretreatment. The excess heat for an overall system composed of thermal pretreatment, anaerobic stabilisation and digester-gas electricity generation is significantly higher than that for a system without pretreatment. Sludge pasteurisation represents an additional advantage of thermal pretreatment. In general, it may be stated that, provided certain conditions are satisfied, thermal pretreatment of sewage sludge prior to anaerobic stabilisation can positively affect the energy balance of the treatment plant.

Effect of chemical coagulation on anaerobic digestibility of organic materials

Previous studies have indicated that coagulants used in wastewater treatment (principally alum or ferric chloride) cause resulting sludges to be less biodegradable in subsequent anaerobic digestion. The objectives of this research were to determine the types of organic materials whose digestibility is most affected by coagulants. Substrates studied included: raw wastewater, activated sludge, three proteins, glycine, cellulose, glucose, butyric acid, and palmitic acid. In general, those substrates whose digestibilities were most affected were those that are insoluble in water and/or are known to form complexes with iron or aluminum. Thus, amino acids, proteins, and long-chain fatty acids were particularly affected, while glucose and butyric acid were not. Cellulose was moderately affected, suggesting that physical enmeshment as well as chemical interactions may be important in determining extent of coagulant effects.