Long term operation of a thermophilic anaerobic reactor : process stability and efficiency at decreasing sludge retention time

The aim of this study was to evaluate the performance of thermophilic sludge digestion at decreasing sludge retention time (SRT) and increasing organic loading rate (OLR), in terms of methane production, effluent stabilisation, hygienisation and dewaterability. Focus was put on determining indicators to help prevent process failure. To this end, a lab-scale reactor was operated for nearly 2 years at 55 °C. Methane production rate was increased (from 0.2 to 0.4-0.6 m³CH₄ m⁻³reactor d⁻¹) by decreasing the SRT from 30 to 15-10 days, while increasing the OLR from 0.5 to 2.5-3.5 kg VS m⁻³reactor d⁻¹. Sludge dewaterability was worsened at SRT below 15 days; while pathogen destruction was always successful. The following concentrations might be used to prevent process failure: VFA C2-C5 (3.7 g COD L⁻¹), acetate (0.6 g L⁻¹), acetate/propionate (0.5), intermediate alkalinity (1.8 g CaCO₃ L⁻¹), intermediate/partial alkalinity (0.9), intermediate/total alkalinity (0.5), CH₄ in biogas (55%)

Dehydrogenase activity as a method for monitoring the composting process

Dehydrogenase enzymatic activity was determined to monitor the biological activity in a composting process of organic fraction of municipal solid waste. Dehydrogenase activity is proposed as a method to describe the biological activity of the thermophilic and mesophilic stages of composting. The maximum dehydrogenase activity was detected at the end of the thermophilic stage of composting, with values within 0.5-0.7 mg g dry matter−1 h−1. Also, dehydrogenase activity can be correlated to static respiration index during the maturation mesophilic stage

The use of respiration indices in the composting process : a review

Respiration is directly related to the metabolic activity of a microbial population. Micro-organisms respire at higher rates in the presence of large amounts of bioavailable organic matter while respiration rate is slower if this type of material is scarce. In the composting process respiration activity has become an important parameter for the determination of the stability of compost. It is also used for the monitoring of the composting process and it is considered an important factor for the estimation of the maturity of the material. A wide range of respirometric protocols has been reported based either on CO₂ production, O₂ uptake or release of heat. The most common methods are those based on O₂ uptake. Respirometric assays are affected by a number of parameters including temperature, humidity, and both incubation and pre-incubation conditions. Results from respirometries are generally expressed as 'respiration indices', most of them with their own units and basis. In consequence, some confusion exists when referring and comparing respiration indices. This is particularly important because current and future legislations define and measure the biological stability of waste on the basis of respiration activity of the material. This paper discusses and compares most common respiration indices currently used

Increasing biogas production by thermal (70ºC) sludge pre-treatment prior to thermophilic anaerobic digestion

The objective of this work was to investigate the effect of a low temperature pre-treatment (70 °C) on the efficiency of thermophilic anaerobic digestion of primary and secondary waste sludge. Firstly, effect of sludge pre-treatment time (9, 24, 48 and 72 h) was evaluated by the increase in volatile dissolved solids (VDSs), volatile fatty acids (VFAs) and biogas production in thermophilic batch tests. Secondly, semi-continuous process performance was studied in a lab-scale reactor (5 L) working at 55 °C and 10 days solid retention time. The 70 °C pre-treatment showed an initial solubilization effect (increasing VDS by almost 10 times after 9 h), followed by a progressive generation of VFA (from 0 to nearly 5 g L⁻¹ after 72 h). Biogas production increased up to 30% both in batch tests and in semi-continuous experiments. Our results suggest that a short period (9 h) low temperature pre-treatment should be enough to enhance methane production through thermophilic anaerobic digestion of sludge

Optimization of the hydrolytic-acidogenic anaerobic digestion stage (55ºC) of sewage sludge : influence of pH and solid content

In conventional single-stage anaerobic digestion processes, hydrolysis is regarded as the rate-limiting step in the degradation of complex organic compounds, such as sewage sludge. Two-stage systems have been proposed to enhance this process. However, so far it is not clear which are the best conditions for a two-stage anaerobic digestion process of sewage sludge, in terms of temperature and hydraulic retention time of each stage. The aim of this work was to determine the optimal conditions for the hydrolytic-acidogenic stage treating real sludge with a high concentration of total solids (40-50 g L⁻¹) and volatile solids (25-30 g L⁻¹), named high concentration sludge. The variables considered for this first stage were: hydraulic retention time (1-4 days) and temperature (55 and 65 °C). Maximum volatile fatty acids generation was obtained at 4 days and 3 days hydraulic retention time for 55 °C and 65 °C, respectively. Consequently, 4 days hydraulic retention time and temperature of 55 °C were set as the working conditions for the hydrolytic-acidogenic stage treating high concentration sludge. The results obtained when operating with high concentration sludge were compared with a low concentration sludge consisting of 17-28 g L⁻¹ total solids and 13-21 g L⁻¹ volatile solids. The effect of decreasing the influent sludge pH, when working at the optimal conditions established, was also evaluated

The use of composting for the treatment of animal by-products : experiments at lab scale

Animal by-products (ABP), containing mainly rabbit and chicken carcasses were composted at laboratory scale. Results indicate that if proper conditions are used, wastes can be successfully composted and stabilised meeting current European hygienisation standards regarding the disposal of this type of wastes. During the process, temperatures above 60 °C were easily reached and maintained for 2 days at least, due to the high energy potential of these materials. However, care must be taken to ensure that these temperatures are reached in the entire reactor to guarantee proper hygienisation of the material. These high temperatures may bring about operational problems such as moisture losses due to very high airflows required for their control. Biological activity indices, such as respiration index (RI) and oxygen uptake rate (OUR) used for the monitoring of the process, were able to indicate potential and actual conditions within the composting reactor, respectively

Full-scale co-composting of hair wastes from the leather manufacturing industry and sewage sludge

A full-scale cocomposting experiment using hair wastes from the leather manufacturing industry and sewage sludge as cosubstrates was carried out with the aim of producing compost that may be used as an organic amendment in agriculture. A 1:1 weight ratio of hair wastes and sewage sludge was used based on experiments at smaller-scale. The resulting mixture was then amended with pruning wastes acting as bulking agent in a 1:1 volumetric ratio (mixture:pruning wastes). The experiment was carried out using the windrow composting technology with a weekly turning frequency. Thermophilic range of temperature was quickly achieved and maintained for more than 8 weeks. This fact ensured the sanitation of the compost according to international requirements. Additionally, the quality of the product obtained expressed in stability terms was very high (Self-heating test grade: V; respiration index: 0.83 mg O₂ per gram of organic matter per hour). The nitrogen content in the final product was also high (5.6%, dry basis). In conclusion, the recycling of hair wastes produced in the leather manufacturing industry using the composting technology is feasible and a valuable organic fertilizer may be obtained

Co-composting of hair waste from the tanning industry with de-inking and municipal wastewater sludges

Production of waste hair in the leather manufacturing industry is increasing every year due to the adoption of hair-save unhairing techniques, leaving the tanners with the problem of coping with yet another solid by-product. Numerous potential strategies for hair utilisation have been proposed. However, the use of hair waste as agricultural fertiliser is one of its most promising applications due to the high nitrogen content of hair. Agricultural value of hair can be increased by composting. This paper deals with the composting of hair from the unhairing of bovine hide. Results indicated that hair cannot be either composted on its own or co-composted with de-inking sludge, a chemical complementary co-substrate. However, good results were obtained when co-composted with raw sludge from a municipal wastewater treatment plant at hair:raw sludge weight ratios 1:1, 1:2 and, 1:4 in lab scale and pilot plant scale composters. In all cases, a more stable product was achieved at the end of the process. Composting in the pilot plant composter was effectively monitored using Static Respiration Indices determined at process temperature at sampling (SRI(T)) and at 37 degrees C (SRI(37)). Notably, SRI(T) values were more sensitive to changes in the biological activity. In contrast, Respiratory Quotient (RQ) values were not adequate to follow the development of the process

Effects of thermal and mechanical pretreatments of secondary sludge on biogas production under thermophilic conditions

Slow degradation of sewage sludge is a disadvantage of anaerobic digestion leading to high sludge retention times in conventional digesters. Hydrolysis has been pointed as the rate-limiting step in this process. Thermophilic anaerobic digestion has been proved effectively to reduce the retention time needed for sludge stabilization. Sludge pretreatment has been also proposed as a strategy to accelerate the hydrolytic step. The effectiveness of high and low temperature thermal pretreatment, ultrasonic and microwave pretreatments in secondary sludge disintegration has been studied by means of the increment in filterable volatile solids to total volatile solids ratio (FVS/TVS) respect to untreated sludge. Increments in this parameter ranging from three-fold for microwave treatment to nine-fold for high temperature thermal treatment have been obtained. Biogas production under thermophilic (55 °C) conditions for treated and untreated secondary sludge has also been evaluated. In spite of the values of FVS/TVS ratio obtained for all the treatments studied, no differences in biogas production were observed when high temperature thermal treatment, ultrasonic and microwave treatments are compared with untreated sludge. A 50% increment in biogas production was observed for low temperature (70 °C) thermally treated sludge. This type of treatment has been pointed as a predigestion step enhancing biological activity of some thermophilic hydrolytic bacteria

In search of a reliable technique for the determination of the biological stability of the organic matter in the mechanical-biological treated waste

The biological stability determines the extent to which readily biodegradable organic matter has decomposed. In this work, a massive estimation of indices suitable for the measurement of biological stability of the organic matter content in solid waste samples has been carried out. Samples from different stages in a mechanical-biological treatment (MBT) plant treating municipal solid wastes (MSW) were selected as examples of different stages of organic matter stability in waste biological treatment. Aerobic indices based on respiration techniques properly reflected the process of organic matter biodegradation. Static and dynamic respirometry showed similar values in terms of aerobic biological activity (expressed as oxygen uptake rate, OUR), whereas cumulative oxygen consumption was a reliable method to express the biological stability of organic matter in solid samples. Methods based on OUR and cumulative oxygen consumption were positively correlated. Anaerobic methods based on biogas production (BP) tests also reflected well the degree of biological stability, although significant differences were found in solid and liquid BP assays. A significant correlation was found between cumulative oxygen consumption and ultimate biogas production. The results obtained in this study can be a basis for the quantitative measurement of the efficiency in the stabilization of organic matter in waste treatment plants, including MBT plants, anaerobic digestion of MSW and composting plants