Effect of pretreatment on acid fermentation of organic solid waste.

The effect of various pretreatment methods on acidogenesis of food waste from a cafeteria was investigated. Thermal, enzymatic, and combined thermal-enzymatic batch pretreatment were conducted. Solubilization of food waste in thermal or enzymatic pretreatment increased with either increasing thermal duration or enzyme dosage, respectively. An optimal condition in thermal or enzymatic pretreatment was 60 min of thermal processing or 0.1% (v/v) of enzyme dosage level, respectively, based on volatile suspended solids (VSS) reduction. In the combined pretreatment, increased thermal duration also increased VSS reduction and the solubilization efficiency was higher than that in only enzymatic or thermal pretreatment. The maximum volatile fatty acids (VFAs) production and the highest VFAs fraction in soluble COD in the acid fermentation were achieved with the food waste after 60 min of thermal processing followed by 0.1% (v/v) enzymatic treatment. Increase in VFAs production of the fermenter was over 380% compared to the control fermenter without any pretreatment.     

Acidogenic fermentation of municipal solid waste and its application to bio-electricity production via microbial fuel cells (MfCs)

Acidogenic fermentation of organic municipal solid waste (MSW) and the bio-electricity production potential from its volatile fatty acid (VFA)-rich leachate using an air-cathode microbial fuel cell (MFC) was investigated in this study. The acidogenic fermentation of 2 kg of MSW has been carried out in a 6 L anaerobic leach-bed reactor (LBR) under mesophilic conditions (30 degrees C). Total production of 92 g VFA expressed as chemical oxygen demand (COD) in 3 L leachate mainly containing acetic, propionic, butyric, and valeric acids has been achieved with manual leachate recirculation and without pH control in 74 days of incubation. Leachate collected on day 32 was used as a feed to an air-cathode MFC after being diluted and supplemented with NaCl or NaHCO3. The maximum power density in the diluted leachate was only 5.9 W/m3, but reached up to 8.6 W/m3 upon the addition of 7 mmol/L NaCl. Increase in coulombic efficiency from 6 to 22% was also observed as a result of NaCl supplementation. On the other hand, NaHCO3 addition did not improve the power output.     

污泥厌氧发酵制氢研究进展

从产氢菌富集和基质污泥预处理两方面探讨不同预处理方式对产氢效能的影响。结果表明:产氢菌富集最为常用的预处理是热处理,其所占比例为30%;其次为酸、碱处理。基质污泥预处理中常用热、酸、碱、灭菌、酶等方法,其中55%预处理方法采用物理预处理。污泥共消化产氢基质主要包括城市固体废弃物和农业固体废弃物,其中,餐厨垃圾作为有效共消化基质备受关注。最后对污泥厌氧发酵制氢的发展方向进行了展望。     

Intracellular biopolymer productions using mixed microbial cultures from fermented POME.

This study aimed to produce polyhydroxyalkanoates (PHAs) from organic wastes by mixed bacterial cultures using anaerobic-aerobic fermentation systems. Palm oil mill effluent (POME) was used as an organic source, which was cultivated in a two-step-process of acidogenesis and acid polymerization. POME was operated in a continuous flow anaerobic reactor to access volatile fatty acids (VFAs) for PHAs production. During fermentation, VFA concentration was produced in the range of 5 to 8 g/L and the COD concentration reduced up to 80% from 65 g/L. The VFA from anaerobic fermentation was then utilised for PHA production using a mixed culture in availability of aerobic bioreactor. Production of PHAs was recorded high when using a high volume of substrates because of the higher VFA concentration. Even though the maximum PHA content was observed at only 40% of the cell dried weight (CDW), their production and performance are significant in mixed microbial culture.     

Thermal pretreatment of the solid fraction of manure: impact on the biogas reactor performance and microbial community.

Application of thermal treatment at 100-140 degrees C as a pretreatment method prior to anaerobic digestion of a mixture of cattle and swine manure was investigated. In a batch test, biogasification of manure with thermally pretreated solid fraction proceeded faster and resulted in the increase of methane yield. The performances of two thermophilic continuously stirred tank reactors (CSTR) treating manure with solid fraction pretreated for 40 minutes at 140 degrees C and non-treated manure were compared. The digester fed with the thermally pretreated manure had a higher methane productivity and an improved removal of the volatile solids (VS). The properties of microbial communities of both reactors were analysed. The specific methanogenic activity (SMA) test showed that both biomasses had significant activity towards hydrogen and formate, while the activity with the VFA - acetate, propionate and butyrate - was low. The kinetic parameters of the VFA conversion revealed a reduced affinity of the microbial community from the CSTR fed with thermally pre-treated manure for acetate, propionate and butyrate. The bacterial and archaeal populations identified by t-RLFP analysis of 16S rRNA genes were found to be identical in both systems. However, a change in the abundance of the species present was detected.     

Optimizing volatile fatty acid production in partial acidogenesis of swine wastewater.

This research has been conducted based on the fact that acetic and butyric acids are favorable substrates for methanogens, and that a low level of propionic acid production during acidogenesis minimizes the inhibition effect on methanogenic growth. Raw swine wastewater was pretreated with ammonia stripping to enhance acidogenesis. The ammonia nitrogen concentration of less than 1.2 g/L did not significantly affect the biochemical acidogenic potential of swine wastewater. For acidogenesis of swine wastewater, a set of experiments were carried out to produce short chain volatile fatty acids (VFA) in laboratory-scale continuously stirred tank reactors. The production of acetic, propionic, and butyric acids associated with simultaneous changes in hydraulic retention time (HRT) and temperature was investigated. Response surface methodology was successfully applied to approximate the responses of the VFA productions. The optimum physiological conditions where the maximum acetic and butyric acids production occurred were 2.4 days HRT at 34 degrees C and 2.1 days HRT at 35 degrees C, respectively. The propionic acid production linearly increased as both HRT and temperature increased.     

Microbial community analysis during continuous fermentation of thermally hydrolysed waste activated sludge

Acidogenic fermentation of thermally hydrolysed waste activated sludge was carried out at laboratory scale in two reactors operated under different hydraulic retention times (HRT). Process performance was assessed in terms of volatile fatty acid (VFA) composition and yield. The diversity of the microbial population was investigated by constructing a 16S rRNA gene library and subsequent phylogenetic analysis of clones. Fluorescence in situ hybridization (FISH) was used to assess the relative abundance of different bacterial groups. Bacteroidetes and Firmicutes were the dominant taxonomic groups representing 93% of the total sequences obtained in the reactor with 4 d HRT. A similar VFA yield (0.4-0.5 g VFA(COD) g SCOD(-1)) was obtained for the HRTs tested (1-4 d), indicating that extended retention times were not useful. Within Firmicutes, Clostridia was the major group detected in the clone sequences. These had close affiliation to Sporanaerobacter acetigenes, suggesting organisms of this group were important for hydrolysis of the protein fraction of the substrate. However, FISH analysis failed to detect the major portion of the bacteria, and this is most likely due to the lack of appropriate probes. This work emphasizes the diversity of fermentative communities, and indicates that more work is needed to identify and detect the important members.     

Online estimation of VFA, alkalinity and bicarbonate concentrations by electrical conductivity measurement during anaerobic fermentation

This paper describes the use of electrical conductivity for measurement of volatile fatty acids (VFA), alkalinity and bicarbonate concentrations, during the anaerobic fermentation process. Two anaerobic continuous processes were studied: the first was a laboratory reactor for hydrogen production from molasses and the second was a pilot process for anaerobic digestion (AD) of vinasses producing methane. In the hydrogen production process, the total VFA concentration, but not bicarbonate concentration, was well estimated from the on-line electrical conductivity measurements with a simple linear regression model. In the methane production process, the bicarbonate concentration and the VFA concentration were well estimated from the simultaneous on-line measurements of pH and electrical conductivity by means of non-linear regression with neural network models. Moreover, the total alkalinity concentration was well estimated from electrical conductivity measurements with a simple linear regression model. This demonstrates the use of electrical conductivity for monitoring the AD processes.     

Applying the least squares method for the titrimetric determination of the concentration of VFA

The titrimetric method is used for on-site measurement of the concentration of volatile fatty acids (VFAs) in anaerobic treatment. In current practice, specific and interpolated pH-volume data points are used to obtain the concentration of VFA by solving simultaneous equations iteratively to convergence (denoted as SEq). Here, the least squares method (LSM) is introduced as an elegant alternative. Known concentrations of VFA (acetic acid and/or propionic acid) ranging from to 200 to 1,000 mg/L were determined using SEq and LSM. Using standard numbers of data points, SEq gave more accurate results compared with LSM. However, results favoured LSM when all data points in the range were included without any interpolation. For model refinement, unit monovalent activity coefficient (f(m) = 1) was found reasonable and arithmetic averages of dissociation constants and molecular weight of 80 mol% acetic acid were recommended in the model for VFA determination of mixtures. An accurate result was obtained with a mixture containing more VFA (butyric acid and valeric acid). In a typical VFA measurement of real anaerobic effluent, a satisfactory result with an error of 14% was achieved. LSM appears to be a promising mathematical model solver for determination of concentration of VFA in the titrimetric method. Validation of LSM in the presence of other electrolytes deserves further exploration.     

水解酸化工艺在氯碱废水预处理中的应用

用水解酸化工艺预处理齐鲁石化氯碱厂所排放的氯碱废水,试验结果表明:水解酸化反应器中COD去除率在20%~34%之间,挥发酸(VFA)质量浓度呈逐步升高的趋势,ρ(BOD)/ρ(COD)的出水值比进水值平均提高了27%,改善了废水的可生化性,多种复杂有机污染物可得到有效降解,为后继的好氧处理提供了有利条件。     

Development and optimisation of VFA driven DEAMOX process for treatment of strong nitrogenous anaerobic effluents.

The recently proposed DEAMOX (DEnitrifying AMmonium OXidation) process combines the anammox reaction with autotrophic denitrifying conditions using sulphide as an electron donor for the production of nitrite from nitrate within an anaerobic biofilm. This paper firstly presents a feasibility study of the DEAMOX process using synthetic (ammonia + nitrate) wastewater where sulphide is replaced by volatile fatty acids (VFA) as a more widespread electron donor for partial denitrification. Under the influent N-NH+4/N-NO3(-) and COD/N-NO3(-) ratios of 1 and 2.3, respectively, the typical efficiencies of ammonia removal were around 40% (no matter whether a VFA mixture or only acetate were used) for nitrogen loading rates (NLR) up to 1236 mg N/l/d. This parameter increased to 80% by increasing the influent COD/N-NO3(-) ratio to 3.48 and decreasing the influent N-NH4 +/N-NO3(-) ratio to 0.29. As a result, the total nitrogen removal increased to 95%. The proposed process was further tested with typical strong nitrogenous effluent such as reject water (total N, 530-566 mg N/l; total COD, 1530-1780 mg/l) after thermophilic sludge anaerobic digestion. For this, the raw wastewater was split and partially ( approximately 50%) fed to a nitrifying reactor (to generate nitrate) and the remaining part ( approximately 50%) was directed to the DEAMOX reactor where this stream was mixed with the nitrified effluent. Stable process performance up to NLR of 1,243 mg N/l/d in the DEAMOX reactor was achieved resulting in 40, 100, and 66% removal of ammonia, NOx(-), and total nitrogen, respectively.     

Effect of primary sludge fermentation products on mass balance for biological treatment.

Laboratory batch experiments were conducted at 20 degrees C to investigate the potential of primary sludge fermentation for the generation of readily biodegradable substrate and to evaluate the effect of fermentation products on mass balance for organic carbon, nitrogen and phosphorus, emphasizing COD fractionation. Fermentation converted between 18 to 30% of the initial volatile suspended solids in the sludge into soluble biodegradable COD. The volatile fatty fraction of the soluble COD was approximately 85% after the fermentation process. The average volatile fatty acid composition in fermentation involved 50% acetic acid, 33% propionic acid, 9% butyric acid and 8% valeric acid, indicating that the most important volatile fatty acid obtained during the biological fermentation process was acetate with more than half of total VFA concentration, which is one of the most important carbon sources for denitrification and biological nutrient removal processes. The recoverable fraction of the fermented sludge supernatant could potentially increase the readily biodegradable COD content of the primary effluent by 5%, together with a potential increase of the soluble nitrogen and phosphorus content by 2%.     

Effect of protein on biohydrogen production from starch of food waste.

This study demonstrated the influence of protein on biohydrogen production from carbohydrates, especially starch, by using different combinations of two model food wastes, rice as starch-rich and soybean residue as protein-rich food waste. It was found the maximum specific hydrogen production potential, 0.99 mol H2/mol initial starch as glucose, and the maximum specific hydrogen production rate, 530 ml H2/h g-VS, occurred at a starch/protein ratio of 1.7. The protein content in the initial food waste not only provided buffering capacity to neutralize the volatile fatty acids as concurrent products but also enhanced the hydrogen production by providing readily available organic nitrogen such as soluble proteins and amino acids to microorganisms.     

ADM1 performance using SS-OFMSW with non-acclimated inoculums

This paper assesses the anaerobic digestion (AD) of the source-sorted organic fraction of municipal solid waste (SS-OFMSW). For this purpose, an experimental programme was implemented involving the operation and monitoring of two bench-scale anaerobic digesters, continuously fed with SS-OFMSW. The mathematical model (ADM1) was then applied to simulate the process of AD of SS-OFMSW. While start-up of the digesters was relatively slow, re-inoculation with cattle manure with effluent dilution reduced the acclimation period and achieved better stability, accommodating a feeding rate at an OLR = 2.39 kg TVS m(-3) day(-1). The high recorded methane gas production rate, reaching (0.1-2.5 m(3) CH(4)/m(3) reactor day), confirms the excellent biodegradability of the type of waste used (SS-OFMSW) and its suitability for AD. Satisfactory simulations of soluble chemical oxygen demand (COD), pH, and methane composition of biogas were obtained, whereas volatile fatty acid (VFA) concentrations in both reactors were over-predicted albeit capturing its general trend.     

Effects of solids concentration, pH and carbon addition on the production rate and composition of volatile fatty acids in prefermenters using primary sewage sludge.

Increasing evidence is emerging that the performance of enhanced biological phosphorus removal (EBPR) systems relies on not only the total amount but also the composition of volatile fatty acids (VFAs). Domestic wastewater often contains limited amounts of VFAs with acetic acid typically being the dominating species. Consequently, prefermenters are often employed to generate additional VFAs to meet the demand for carbon by EBPR and/or denitrification processes. Limited knowledge is currently available on the effects of operational conditions on the production rate and composition of VFAs in prefermenters. In this study, a series of controlled batch experiments were conducted with sludge from a full-scale prefermenter to determine the impact of solids concentration, pH and addition of molasses on prefermentation processes. It was found that an increase in solids concentration enhanced total VFA production with an increased propionic acid fraction. The optimal pH for prefermentation was in the range of 6-7 with significant productivity loss when pH was below 5.5. Molasses addition significantly increased the production of VFAs particularly the propionic acid. However, the fermentation rate was likely limited by the biological activity of the sludge rather than by the amount of molasses added.     

Increased biogas production in a wastewater treatment plant by anaerobic co-digestion of fruit and vegetable waste and sewer sludge - a full scale study

Anaerobic digestion is a well established technology for the reduction of organic matter and stabilization of wastewater. Biogas, a mixture of methane and carbon dioxide, is produced as a useful by-product of the process. Current solid waste management at the city of Prince George is focused on disposal of waste and not on energy recovery. Co-digestion of fresh fruit and vegetable waste with sewer sludge can improve biogas yield by increasing the load of biodegradable material. A six week full-scale project co-digesting almost 15,000 kg of supermarket waste was completed. Average daily biogas production was found to be significantly higher than in previous years. Digester operation remained stable over the course of the study as indicated by the consistently low volatile acids-to-alkalinity ratio. Undigested organic material was visible in centrifuged sludge suggesting that the waste should have been added to the primary digester to prevent short circuiting and to increase the hydraulic retention time of the freshly added waste.     

Control of C/N ratio for butyric acid production from textile wastewater sludge by anaerobic digestion

Increasing textile wastewaters and their biotreatment byproduct-waste activated sludge are serious pollution problems. Butyric acid production from textile wastewater sludge by anaerobic digestion at different C/N ratios was investigated. Adding starch to textile wastewater sludge with a C/N ratio of 30 increased the butyric acid concentration and percentage accounting for total volatile fatty acids (TVFAs) to 21.42 g/L and 81.5%, respectively, as compared with 21.42 g/L and 10.6% of textile wastewater sludge alone. The maximum butyric acid yield (0.45 g/g VS), conversion rate (0.74 g/g VS(digest)) and production rate (2.25 g/L d) was achieved at a C/N ratio of 30. The biological toxicity of textile wastewater sludge also significantly decreased after the anaerobic digestion. The study indicated that the anaerobic co-digestion of textile wastewater sludge and carbohydrate-rich waste with appropriate C/N ratio is possible for butyric acid production.     

Sulfide production and wastewater quality in pressure mains

An empirical model for predicting sulfide production in pressure mains (Hvitved-Jacobsen et al., 1988) was evaluated and modified based on results obtained from two intercepting pressure mains located in the Northern part of Jutland, Denmark. Mass balances in pipe influent and effluent were made for volatile fatty acids, VFA (formate, acetate, propionate and butyrate), dissolved COD, DOC and sulfide and biofilm surface rate for sulfide and organic matter were calculated. Relatively high sulfide formation rates were observed at low temperatures (5–12°C). The sulfide production rate strongly depended on wastewater quality in terms of VFA and dissolved carbohydrate concentration. Based on these two sets of observations — wastewater quality and temperature — the original empirical model was modified.     

Phosphorus release and uptake during start-up of a covered and non-aerated sequencing batch reactor with separate feeding of VFA and sulfate

This study explored a sulfur cycle-associated biological phosphorus (P) removal process in a covered and non-aerated sequencing batch reactor (SBR) fed with volatile fatty acid (VFA) and sulfate separately. During the 60-day start-up, both phosphate release and uptake rates increased, while poly-phosphate cyclically increased and decreased accordingly. The P-release and P-uptake rates were associated with VFA uptake and sulfate reduction. The average ratio of potassium to phosphate during the P-uptake and P-release was also determined to be 0.29-0.31 mol K/mol P, which is close to a reported value (0.33) for biological phosphorus removal. All this evidence confirmed there was biological P removal in this reactor, in which metabolism could be different from conventional biological P removal.     

A novel process for organic acids and nutrient recovery from municipal wastewater sludge.

In this paper, a novel process for organic acids and nutrient recovery from municipal sludge was introduced and evaluated based on laboratory-scale studies. An economical estimation for its practical application was also performed by mass balance in a full-scale plant (Q=158,000 m3 d(-1)). This novel process comprises an upflow sludge blanket-type high performance elutriated acid fermenter (5d of SRT) for organic acids recovery followed by an upflow-type crystallisation (3 h of HRT) reactor using waste lime for nutrient recovery. In the system, the fermenter is characterised by thermophilic (55 degrees C) and alkaline conditions (pH 9), contributing to higher hydrolysis/acidogenesis (0.18 g VFA(COD) g(-1) VSS(COD), 63.3% of VFA(COD)/COD produced, based on sludge characteristics of the rainy season) and pathogen-free stabilised sludge production. It also provides the optimal condition for the following crystallisation reactor. In the process, the waste lime, which is an industrial waste, can be used for pH control and cation (Ca and Mg) sources for crystallisation reaction. A cost estimation for full-scale application demonstrates that this process has economic benefits (about 67 dollars per m3 of wastewater except for the energy expense) even in the rainy season.