Thermophilic Digestion System Research Articles

Effects of reactive oxygen species scavengers on thermophilic micro-aerobic digestion for sludge stabilization

In thermophilic digestion systems, sludge stabilization may be adversely impacted by high concentrations of ammonia nitrogen or the rapid accumulation of fatty acids; however, few studies have focused on the mitigation of the inhibition of reactive oxygen species (ROS). In this study, fulvic acids or tea polyphenols were introduced to a thermophilic digestion system and the effects of ROS scavengers on sludge stabilization were investigated. As fulvic acids or tea polyphenols were added to sludge, they reacted with metal cations, such as Cu2+ and Zn2+, to form stable complexes that enabled active metal ions to be transported into cells to enhance the oxidase activities. Therefore, the digestion systems presented a lower O2•- content compared with that of a control group with no additive. Both fulvic acids and tea polyphenols mitigated the adverse effects of the ROS and enhanced the reduction of volatile solids (VS), however, fulvic acids better facilitated the sludge stabilization. The optimal dosage was 0.3% of the total solids as fulvic acids were added to the sludge every 48 h. The VS reduction in the digester reached 38.2% at 14 d, which was clearly higher than that in the control group. The pathway of ROS scavengers affecting sludge stabilization was proposed, and it may be helpful to gain deeper insight into the characteristics of thermophilic digestion processes as well as the mechanism of sludge stabilization.

The synthetic effect on volatile fatty acid disinhibition and methane production enhancement by dosing FeCl3in a sludge thermophilic anaerobic digestion system

A dosage gradient of FeCl3was adopted and 9.92 mg Fe per g DS was favorable for the disinhibition of VFAs in sludge thermophilic digestion system.

Evaluation of anaerobic digestion processes for short sludge-age waste activated sludge combined with anammox treatment of digestate liquor.

The need to reduce energy input and enhance energy recovery from wastewater is driving renewed interest in high-rate activated sludge treatment (i.e. short hydraulic and solids retention times (HRT and SRT, respectively)). This process generates short SRT activated sludge stream, which should be highly degradable. However, the evaluation of anaerobic digestion of short SRT sludge has been limited. This paper assesses anaerobic digestion of short SRT sludge digestion derived from meat processing wastewater under thermophilic and mesophilic conditions. The thermophilic digestion system (55°C) achieved 60 and 68% volatile solids destruction at 8 day and 10 day HRT, respectively, compared with 50% in the mesophilic digestion system (35°C, 10 day HRT). The digestion effluents from the thermophilic (8-10 day HRT) and mesophilic systems were stable, as assessed by residual methane potentials. The ammonia rich sludge dewatering liquor was effectively treated by a batch anammox process, which exhibited comparable nitrogen removal rate as the tests using a control synthetic ammonia solution, indicating that the dewatering liquor did not have inhibiting/toxic effects on the anammox activity.

The one-stage autothermal thermophilic aerobic digestion for sewage sludge treatment: Effects of temperature on stabilization process and sludge properties

One-stage autothermal thermophilic aerobic digestion (ATAD) has potential application in wastewater treatment plants due to its simplicity and small area occupation. Aimed to investigate the effects of digestion temperature on stabilization process and sludge properties, batch experiments were carried out at 35, 45, 55, and 65°C in simulated one-stage ATAD digesters with effective volume of 5L. Thermophilic digestion systems at 45 and 55°C attained more rapid volatile solids (VS) removal than a mesophilic digester at 35°C, however raising digestion temperature to 65°C adversely affected sludge stabilization. At the end of the experiment (i.e. 552h), VS removals in the 35, 45, 55 and 65°C digesters were 33.6%, 42.9%, 45.0% and 24.5%, respectively. Digestion temperature has significant effect on VS reduction. As volatile fatty acids (VFA) accumulated rapidly from 72 to 264h, acetic and propionic acids became the main VFA constituents in the thermophilic digester with digestion temperature of 55°C, however only acetic acid was a major constituent of VFA in the 35°C digester. After digestion for 552h, characteristic absorption bands of carboxyl group, aliphatic chains, carboxylate group and amine group were significantly weakened in the 35 and 55°C digesters. VS concentration is well fitted by an exponential decay function over a range of 35–55°C.

Digestion Performance of Various Combinations of Thermophilic and Mesophilic Sludge Digestion Systems

Various combinations of single- and multi-stage anaerobic and aerobic-anaerobic digestion systems were studied to evaluate their solids reduction potential with capabilities to control sulfur-based biosolids odor compounds. All the multi-stage digestion systems removed more volatile solids than the single-stage anaerobic digestion systems, even at the same overall retention time. However, digestion systems with mesophilic digestion as the final stage showed a much lower headspace organic sulfur content in the dewatered biosolids than the systems with thermophilic digestion as the final stage. This observation leads to the conclusion that placing a mesophilic anaerobic digestion system at the end of multi-stage digestion systems will enable greater sulfur-based odor reduction from dewatered biosolids with greater solid reduction than single-stage mesophilic or thermophilic digestion systems.

Digestion Performance of Various Combinations of Thermophilic and Mesophilic Sludge Digestion Systems

Various combinations of single- and multi-stage anaerobic and aerobic-anaerobic digestion systems were studied to evaluate their solids reduction potential with capabilities to control sulfur-based biosolids odor compounds. All the multi-stage digestion systems removed more volatile solids than the single-stage anaerobic digestion systems, even at the same overall retention time. However, digestion systems with mesophilic digestion as the final stage showed a much lower headspace organic sulfur content in the dewatered biosolids than the systems with thermophilic digestion as the final stage. This observation leads to the conclusion that placing a mesophilic anaerobic digestion system at the end of multi-stage digestion systems will enable greater sulfur-based odor reduction from dewatered biosolids with greater solid reduction than single-stage mesophilic or thermophilic digestion systems.

Impact of sludge thickening on energy recovery from anaerobic digestion

This paper presents energy balances for various digestion systems, which include single mesophilic digestion, single thermophilic digestion, two-stage thermophilic-mesophilic digestion system and systems at elevated solids content in sludge. On the basis of a sludge flow containing 30 tons TS/day (equivalent to a 100 ML/d WWTP plant) it was shown that a two-stage thermophilic-mesophilic digestion system generated more available energy than single mesophilic digestion and single thermophilic digestion systems. Sludge thickening offered the greatest amount of available energy; however that energy surplus was offset by the cost of thickening. After the cost of thickening was converted into equivalent energy units it was shown that the price of energy is important in calculation of equivalent energy units related to operation of the thickening plant. Sludge thickening may be beneficial from energy view point compared to conventional mesophilic digestion when price of energy exceeds dollars 0.08 CAN kW-hr.

Heat and energy requirements in thermophilic anaerobic sludge digestion

The heating requirements of the thermophilic anaerobic digestion process were studied. Biogas production was studied in laboratory experiments at retention times from 1 to 10 days. The data gathered in the experiments was then used to perform a heat and energy analysis. The source of heat was a conventional CHP unit system. The results showed that thermophilic digestion is much faster than mesophilic digestion and therefore produces more biogas in a shorter time or at smaller digester volumes. The major part of the heating requirements consisted of sludge heating. The heat losses of the digester were only 2–8% of the sludge heating requirements. The heating requirements in thermophilic digestion are about twice those of mesophilic digestion. Therefore a CHP unit system cannot cover all of the needs for successful operation of thermophilic digestion. Heat regeneration was introduced as a solution. Heat is regenerated from the sludge outflow at a temperature of 50–55 °C and transferred to the cold inflow sludge at a temperature of 11 °C. Enough heat is regenerated in a conventional counter flow heat exchanger to bring the thermophilic process to the same level as the mesophilic one. Considering the smaller digester volumes and the relatively small investment in the regenerative equipment, the construction of thermophilic digestion systems may be a very good alternative to conventional mesophilic sludge digestion systems.

OPERATIONAL IMPROVEMENTS FROM START-UP OF OWASA'S CLASS-A THERMOPHILIC ANAEROBIC DIGESTION SYSTEM

OPERATIONAL IMPROVEMENTS FROM START-UP OF OWASA'S CLASS-A THERMOPHILIC ANAEROBIC DIGESTION SYSTEMThe thermophilic digestion system at the Mason Farm WWTP in Chapel Hill North Carolina was brought on line in August 2000. This unique facility has the ability to produce class-A biosolids. The 12-mgd Mason Farm WWTP is operated by the Orange Water and Sewer Authority (OWASA) and treats an average of 8.5 mgd.Author(s)John L. WillisWalter GottschalkSourceProceedings of the Water Environment FederationSubjectSession 45 - Residuals and Biosolids Management Symposium: Class A Beneficial ReuseDocument typeConference PaperPublisherWater Environment FederationPrint publication date Jan, 2001ISSN1938-6478SICI1938-6478(20010101)2001:12L.302;1-DOI10.2175/193864701790864935Volume / Issue2001 / 12Content sourceWEFTECFirst / last page(s)302 - 308Copyright2001Word count64