Treatment of the biodegradable fraction of used disposable diapers by co-digestion with waste activated sludge

The results presented in this paper are part of a project aimed at designing an original solution for the treatment of used disposable diapers permitting the recycling of materials and the recovery of energy. Diapers must be collected separately at source and transported to an industrial facility to undergo special treatment which makes it possible to separate plastics and to recover a biodegradable fraction (BFD) made up mainly of cellulose. The methane yield of BFD was measured and found to be 280 ml CH4/g VSfed on average. 150 kg of dry BFD can be retrieved from the treatment of one ton of used disposable diapers, representing an energy potential of about 400 kW h of total energy or 130 kW h of electricity. As the treatment process for used diapers requires very high volumes of water, the setting up of the diaper treatment facility at a wastewater treatment plant already equipped with an anaerobic digester offers the advantages of optimizing water use as well as its further treatment and, also, the anaerobic digestion of BFD. The lab-scale experiments in a SBR showed that BFD co-digestion with sewage sludge (38% BFD and 62% waste activated sludge on volatile solids basis) was feasible. However, special attention should be paid to problems that might arise from the addition of BFD to a digester treating WAS such as insufficient mixing or floating particles leading to the accumulation of untreated solids in the digester.

Effect of the addition of fatty by-products from the refining of vegetable oil on methane production in co-digestion

The purpose of this work was to investigate the effects of the addition of by-products from the refining of vegetable oil on the behavior of co-digestion reactors treating a mixture of grass, cow dung and fruit and vegetable waste. Three by-products were used: one soapstock, one used winterization earth and one skimming of aeroflotation of the effluents. Three 15 l reactors were run in parallel and fed five times a week. In a first phase of 4 weeks, the three reactors were fed with the co-digestion substrates alone (grass, cow dung and fruit and vegetable waste) at an organic loading rate (OLR) of 1.5 g VS/kg d (VS: volatile solids). Then, a different by-product from the refining of oil was added to the feed of each reactor at an OLR of 0.5 g VS/kg d, generating a 33% increase in the OLR. The results show that the addition of by-products from the refining of oil is an efficient way of increasing the methane production of co-digestion reactors thanks to high methane yield of such by-products (0.69-0.77 l CH(4)/g VS loaded). In fact, in this work, it was possible to raise the methane production of the reactors by about 60% through a 33% increase in the OLR thanks to the addition of the by-products from the refining of vegetable oil.

Treatment of fatty solid waste from the meat industry in an anaerobic sequencing batch reactor: start-up period and establishment of the design criteria

An anaerobic sequencing batch reactor (AnSBR) was used to treat the dissolved air flotation skimmings from a cooked pork meat plant. During the start-up period, the reactor was operated in fed-batch mode for 25 days and 7 batches were treated. The SBR was inoculated with sludge taken from a reactor treating distillery vinasse. The results showed that this kind of sludge is a very good source of inoculum for digesters treating residues with a high content in fats and long-chain fatty acids because it was able to adapt very rapidly to the new substrate and, from the second batch on, the sludge was already able to metabolize the fatty residue at quite high rates. The AnSBR was then operated with 5 batches per week for 110 days and the quantity of VS added per batch was regularly increased until the maximum treatment capacity of the reactor (i.e. maximum loading rate) was reached. The maximum organic loading rates were found to be 0.16 g VS/g VSS d, or 0.224 g VS/g VSS.batch when the reactor is fed 5 times a week. The biodegradability of the skimmings was very high, with more than 97% of TS removal, and the methane production was 880+/-90 mL of methane/g of VS(added).

Anaerobic digestion of residues from production and refining of vegetable oils as an alternative to conventional solutions

The purpose of this work was to study the anaerobic digestion of by-products generated during the production and refining of oil with the objective of proposing an alternative solution (methanisation) to the conventional solutions while reducing the energy consumption of fossil origin on refinery sites. The production of sunflower oil was taken as example. Glycerine from the production of biodiesel was also included in this study. The results show that glycerine has a high potential for methanisation because of its high methane potential (465 ml CH4/g VS) and high metabolization rates (0.42 g VS/g VSS.d). The use of oil cake as substrate for anaerobic digestion is not interesting because it has a low methane potential of 215 ml CH4/g VS only and because it is easily recovered in animal feed. Six residues have quite a high methane potential (465 to 850 ml CH4/g VS) indicating a good potential for anaerobic digestion. However, they contain a mixture of rapidly and slowly biodegradable organic matter and the loading rates must remain quite low (0.03 to 0.09 g VS/g VSS.d) to prevent any accumulation of slowly biodegradable solids in the digesters.

Characterization of the activated sludge and of the operating conditions of six SBR treating dairy effluent and operated at industrial scale

The sludge from six SBRs treating dairy effluent and located at same geographical location, in North East of France, were collected to study their characteristic behavior. The six plants were designed and constructed by the same manufacturer and are working under quite similar operating conditions. The objective of the study was to observe if any similarity existed in the characteristics of the sludge collected from the SBRs. The sludge was characterized for morphological properties (filament index, floc size), settling, compressibility, suspended solids (SS) concentration. The sludge from each plant was different from the others in most of the characteristics. One sludge out of six (sludge G) was completely different from the others with a very degraded structure and low discrete settling and compression. This reactor was not working fully satisfactorily with a too high COD at outlet, probably because this SBR was undergoing repetitive overloading linked to a very bad recovery of the whey by the cheese maker. The five other SBRs were working fully satisfactorily but the characteristics of the five sludges were quite different from one sludge to another. The size of the flocs seemed to be the only parameter measured which could be correlated to the settling characteristics of the sludge. The sludge characteristics and the parameter correlations were also compared with that of municipal activated sludge and were found to be very different.

Evaluation of a four year experience with a fully instrumented anaerobic digestion process

For several years, a 1 m3 fixed bed anaerobic digestion process has been operated for the treatment of distillery vinasses. This reactor has been fully instrumented with the following variables available on-line: pH, temperature, liquid and gas flow rates, gas composition (i.e., CH4, CO2 and H2), concentration of bicarbonate, chemical oxygen demand, total organic carbon, volatile fatty acids and partial and total alkalinity, these last four variables being measured twice by different techniques (i.e., using a TOC analyzer, a titrimetric sensor and an infrared spectrometer). The purpose of this paper is to compare the respective benefits of advanced instrumentation for the monitoring of wastewater treatment processes in general, and for anaerobic digestion in particular. It will also provide some statistical analysis of the time required to operate a fully instrumented wastewater treatment process. It is indeed well admitted in the literature that instrumentation is usually the main limitation step for using closed-loop control. However, it is our opinion that, in the near future, this situation will change. This point is discussed based on our four years practical experience.

Treatment of winery wastewater by an anaerobic sequencing batch reactor

Treatment of winery wastewater was investigated using an anaerobic sequencing batch reactor (ASBR). Biogas production rate was monitored and permitted the automation of the bioreactor by a simple control system. The reactor was operated at an organic loading rate (ORL) around 8.6 gCOD/L.d with soluble chemical oxygen demand (COD) removal efficiency greater than 98%, hydraulic retention time (HRT) of 2.2 d and a specific organic loading rate (SOLR) of 0.96 gCOD/gVSS.d. The kinetics of COD and VFA removal were investigated for winery wastewater and for simple compounds such as ethanol, which is a major component of winery effluent, and acetate, which is the main volatile fatty acid (VFA) produced. The comparison of the profiles obtained with the 3 substrates shows that, overall, the acidification of the organic matter and the methanisation of the VFA follow zero order reactions, in the operating conditions of our study. The effect on the gas production rate resulted in two level periods separated by a sharp break when the acidification stage was finished and only the breaking down of the VFA continued.

The anaerobic SBR process: basic principles for design and automation

This study has determined the purification performance and the basic principles for the design of an anaerobic SBR (ASBR) to be used to treat wastewater generated in the food industries. Two ASBR's were set up and one fed with a slaughterhouse effluent at low concentration, the other with concentrated dairy wastewater. The maximum loading rate applied should not exceed 4.5 g of COD/L/day for the dilute effluent and 6 g of COD/L/day for the concentrated effluent. At higher loading rates, the reactors become difficult to operate, mainly because of sludge removal problems, and purification efficiency declines. A detailed study of the kinetics (TOC, VFA, rate of biogas production) throughout one treatment cycle led to the development of a simple control strategy based on the monitoring of the biogas production rate which was then applied to the reactor treating the dairy wastewater. After automation, the reactor worked free of problems at an average pollution load of 5.4 g of COD/L/day.