Source-separated Organic Household Waste Research Articles

Assessment of a combined dry anaerobic digestion and post-composting treatment facility for source-separated organic household waste, using material and substance flow analysis and life cycle inventory

The fate of total solids, volatile solids, total organic carbon, fossil carbon, biogenic carbon and 17 substances (As, Ca, CaCO3, Cd, Cl, Cr, Cu, H, Hg, K, Mg, N, Ni, O, P, Pb, S, Zn) in a combined dry anaerobic digestion and post-composting facility were assessed. Mass balances showed good results with low uncertainties for non-volatile substances, while balances for nitrogen, carbon, volatile solids and total organic carbon showed larger but reasonable uncertainties, due to volatilisation and emissions into the air. Material and substance flow analyses were performed in order to obtain transfer coefficients for a combined dry anaerobic digestion and post-composting facility. All metals passed through the facility and ended up in compost or residues, but all concentrations of metals in the compost complied with legislation. About 23% of the carbon content of the organic waste was transferred to the biogas, 24% to the compost, 13% to residues and 40% into the atmosphere. For nitrogen, 69% was transferred to the compost, 10% volatilised to the biofilter, 11% directly into the atmosphere and 10% to residues. Finally, a full life cycle inventory was conducted for the combined dry anaerobic digestion and post-composting facility, including waste received, fuel consumption, energy use, gaseous emissions, products, energy production and chemical composition of the compost produced.

Global warming potential of material fractions occurring in source-separated organic household waste treated by anaerobic digestion or incineration under different framework conditions

This study compared the environmental profiles of anaerobic digestion (AD) and incineration, in relation to global warming potential (GWP), for treating individual material fractions that may occur in source-separated organic household waste (SSOHW). Different framework conditions representative for the European Union member countries were considered. For AD, biogas utilisation with a biogas engine was considered and two potential situations investigated – biogas combustion with (1) combined heat and power production (CHP) and (2) electricity production only. For incineration, four technology options currently available in Europe were covered: (1) an average incinerator with CHP production, (2) an average incinerator with mainly electricity production, (3) an average incinerator with mainly heat production and (4) a state-of-the art incinerator with CHP working at high energy recovery efficiencies. The study was performed using a life cycle assessment in its consequential approach. Furthermore, the role of waste-sorting guidelines (defined by the material fractions allowed for SSOHW) in relation to GWP of treating overall SSOHW with AD was investigated. A case-study of treating 1tonne of SSOHW under framework conditions in Denmark was conducted. Under the given assumptions, vegetable food waste was the only material fraction which was always better for AD compared to incineration. For animal food waste, kitchen tissue, vegetation waste and dirty paper, AD utilisation was better unless it was compared to a highly efficient incinerator. Material fractions such as moulded fibres and dirty cardboard were attractive for AD, albeit only when AD with CHP and incineration with mainly heat production were compared. Animal straw, in contrast, was always better to incinerate. Considering the total amounts of individual material fractions in waste generated within households in Denmark, food waste (both animal and vegetable derived) and kitchen tissue are the main material fractions allowing GWP mitigation with AD when it is compared to incineration. The inclusion of other material fractions in SSOHW sorting guidelines may be considered of less importance.

Characterisation of the biochemical methane potential (BMP) of individual material fractions in Danish source-separated organic household waste

This study is dedicated to characterising the chemical composition and biochemical methane potential (BMP) of individual material fractions in untreated Danish source-separated organic household waste (SSOHW). First, data on SSOHW in different countries, available in the literature, were evaluated and then, secondly, laboratory analyses for eight organic material fractions comprising Danish SSOHW were conducted. No data were found in the literature that fully covered the objectives of the present study. Based on laboratory analyses, all fractions were assigned according to their specific properties in relation to BMP, protein content, lipids, lignocellulose biofibres and easily degradable carbohydrates (carbohydrates other than lignocellulose biofibres). The three components in lignocellulose biofibres, i.e. lignin, cellulose and hemicellulose, were differentiated, and theoretical BMP (TBMP) and material degradability (BMP from laboratory incubation tests divided by TBMP) were expressed. Moreover, the degradability of lignocellulose biofibres (the share of volatile lignocellulose biofibre solids degraded in laboratory incubation tests) was calculated. Finally, BMP for average SSOHW composition in Denmark (untreated) was calculated, and the BMP contribution of the individual material fractions was then evaluated. Material fractions of the two general waste types, defined as “food waste” and “fibre-rich waste,” were found to be anaerobically degradable with considerable BMP. Material degradability of material fractions such as vegetation waste, moulded fibres, animal straw, dirty paper and dirty cardboard, however, was constrained by lignin content. BMP for overall SSOHW (untreated) was 404mL CH4 per g VS, which might increase if the relative content of material fractions, such as animal and vegetable food waste, kitchen tissue and dirty paper in the waste, becomes larger.

Evaluation of a new pulping technology for pre-treating source-separated organic household waste prior to anaerobic digestion

A new technology for pre-treating source-separated organic household waste prior to anaerobic digestion was assessed, and its performance was compared to existing alternative pre-treatment technologies. This pre-treatment technology is based on waste pulping with water, using a specially developed screw mechanism. The pre-treatment technology rejects more than 95% (wet weight) of non-biodegradable impurities in waste collected from households and generates biopulp ready for anaerobic digestion. Overall, 84–99% of biodegradable material (on a dry weight basis) in the waste was recovered in the biopulp. The biochemical methane potential for the biopulp was 469±7mL CH4/g ash-free mass. Moreover, all Danish and European Union requirements regarding the content of hazardous substances in biomass intended for land application were fulfilled. Compared to other pre-treatment alternatives, the screw-pulping technology showed higher biodegradable material recovery, lower electricity consumption and comparable water consumption. The higher material recovery achieved with the technology was associated with greater transfer of nutrients (N and P), carbon (total and biogenic) but also heavy metals (except Pb) to the produced biomass. The data generated in this study could be used for the environmental assessment of the technology and thus help in selecting the best pre-treatment technology for source separated organic household waste.

Fungal survival during anaerobic digestion of organic household waste

Anaerobic digestion of organic waste yields energy rich biogas and retains nutrients (N, P, K, S, etc.) in a stabilised residue. For the residue to be used as a soil fertiliser, it must be free from pollutants and harmful microorganisms. Fungal survival during sanitation and anaerobic treatment of source-separated organic household waste and during aerobic storage of the residue obtained was investigated. Decimal reduction times were determined for inoculated fungi ( Aspergillus flavus and Aspergillus fumigatus, Penicillium roqueforti, Rhizomucor pusillus, Thermoascus crustaceus and Thermomyces lanuginosus). Several different fungal species were found after waste sanitation treatment (70 °C, 1 h), with Aspergillus species dominating in non-inoculated waste. Anaerobic waste degradation decreased the diversity of fungal species for processes run at both 37 and 55 °C, but not total fungal colony forming units. Fungi surviving the mesophilic anaerobic digestion were mainly thermotolerant Talaromyces and Paecilomyces species. T. crustaceus and T. lanuginosus were the only inoculated fungi to survive the thermophilic anaerobic degradation process. Aerobic storage of both types of anaerobic residues for one month significantly decreased fungal counts.

Microbial Community Growth and Utilization of Carbon Constituents During Thermophilic Composting at Different Oxygen Levels

Composting is characterized by dramatic changes in microbial community structure, to a high extent driven by changes in temperature and in the composition of the organic substrate. This study focuses on the interrelationships between decomposition of major classes in the organic material and dynamics in microbial populations during thermophilic composting of source-separated organic household waste. Experiments were performed in a 200-L laboratory reactor at 16, 2.5, and 1% O(2) in the compost atmosphere. Major classes of carbon constituents were analyzed by chemical methods, and the microbial biomass and community structure determined by fatty acid analyses with phospholipid fatty acids (PLFA) and total ester-linked fatty acids (EL) methods. At all three O(2) levels, the process was characterized by a rapid increase in microbial activity and biomass in the early thermophilic phase, although this period was delayed at the lower O(2) concentrations. Starch and fat were the main substrates utilized at all three O(2) levels during this period. The depletion of the starch fraction coincided with the beginning of a microbial biomass decrease, suggesting that starch is an important carbon substrate for the growth of thermophilic microorganisms during composting. Growth yields in the microbial community based on consumption of major carbon constituent classes in the high-activity period fell between 22 and 28%. Multivariate statistical analysis of changes in fatty acid composition revealed small, but statistically significant differences in the microbial community succession. At 16% O(2), 10 Me fatty acids from Actinomycetes and cyclopropyl fatty acids (from Gram-negative bacteria) became more important with time, whereas 18:1 omega 7t was characteristic at 2.5 and 1% O(2), indicating a more stressed bacterial community at the lower O(2) concentrations. Although adequate composting was achieved at O(2) levels as low as 2.5 and 1%, it is not recommended to compost at such low levels in large-scale systems, because the heterogeneous gas transport through the material in these systems might lead to anaerobic conditions and inefficient composting.

Method for determination of methane potentials of solid organic waste

A laboratory procedure is described for measuring methane potentials of organic solid waste. Triplicate reactors with 10 grams of volatile solids were incubated at 55 °C with 400 ml of inoculum from a thermophilic biogas plant and the methane production was followed over a 50-day period by regular measurements of methane on a gas chromatograph. The procedure involves blanks as well as cellulose controls. Methane potentials have been measured for source-separated organic household waste and for individual waste materials. The procedure has been evaluated regarding practicality, workload, detection limit, repeatability and reproducibility as well as quality control procedures. For the source-separated organic household waste a methane potential of 495 ml CH 4/g VS was found. For fat and oil a lag-phase of several days was seen. The protein sample was clearly inhibited and the maximal methane potential was therefore not achieved. For paper bags, starch and glucose 63, 84 and 94% of the theoretical methane potential was achieved respectively. A detection limit of 72.5 ml CH 4/g VS was calculated from the results. This is acceptable, since the methane potential of the tested waste materials was in the range of 200–500 ml CH 4/g VS. The determination of methane potentials is a biological method subject to relatively large variation due to the use of non-standardized inoculum and waste heterogeneity. Therefore, procedures for addressing repeatability and reproducibility are suggested.

Assessment of sampling and chemical analysis of source-separated organic household waste

The quality of the waste sampling procedure and chemical analysis was evaluated in a research program on characterization of organic waste obtained after disc screening of source-separated organic household waste. The sampling procedures focused on a truckload of waste and involved several steps of subsampling including shredding, mixing, blending, high-speed-blending, drying and milling prior to analysis of the organic waste with respect to ash content, crude fibers, crude fat, crude protein, sugar, starch, enzyme-digestible organic matter, P, N, C, H, S and calorific value. The statistical evaluation of the procedures involved 10 samples of the same truckload of waste obtained by splitting the sample at each level in the procedure according to a staggered, incomplete nested statistical design. Furthermore, one sample was analysed six times over a period of approximately one year. The statistical evaluation showed that no single step in the sampling procedure contributed with excessive variance and that the variance caused by the sampling procedure was approximately the same as the variance in the chemical analysis observed over a year. The variance varied with the analytical parameter but for most parameters the uncertainty was satisfactorily low (of the order of 3–10% expressed as the relative standard deviation, which is considered to be satisfactory for waste characterization).

Comparison of signature lipid methods to determine microbial community structure in compost

The microbial community structure changes substantially during the composting process and simple methods to follow these changes can potentially be used to estimate compost maturity. In this study, two such methods, the microbial identification (MIDI) method and the ester-linked (EL) procedure to determine the composition of long-chain fatty acids, were applied to compost samples of different age. The ability of the two methods to describe the microbial succession was evaluated by comparison with phospholipid fatty acid (PLFA) analysis on the same samples. Samples were taken from a 200-l laboratory compost reactor, treating source-separated organic household waste. During the initial stages of the process, the total concentration of fatty acids in compost samples treated with the EL and MIDI methods was many times higher than with the PLFA method. This was probably due to the presence of fatty acids from the organic material in the original waste. However, this substantial difference between PLFA and the other two methods was not found later in composting. Although the PLFA method gave the most detailed information about the growth and overall succession of the microbial community, the much simpler MIDI and EL methods also successfully described the shift from the initially dominating straight chain fatty acids to iso- and anteiso branched, 10 Me branched and cyclopropane fatty acids in the later stages of the process. Thus, the MIDI and EL extraction methods appear to be suitable for analysis of microbial FAME profiles in compost, particularly in the later stages of the process.

Influence of rates and timing of biowaste compost application on rye yield and soil nitrate levels

Beneficial use of biowaste compost depends on identifying an application strategy that optimizes crop yield and minimizes nitrate leaching, soil compaction and labor. The effects of applying the same total quantity of biowaste compost in different doses and in different intervals were assessed in a randomized 5–year field trial with permanent rye. Biowaste compost (from source-separated organic household waste and yard trimmings) was applied either at 20 t ha –1 (fresh matter) annually, or at 40 t ha –1 every second year, or at 60 t ha –1 every third year, or at 60 t ha –1 in two consecutive years, or at a single dose of 70 t ha –1. The application of 40 t ha –1 in 2-year-intervals resulted in slightly higher rye yields than the other treatments. However, the differences between the treatments were not statistically significant. In the treatment receiving 20 t ha –1 annually, yields increased after 4 years for the first time. The baking quality of the rye was good in all treatments. The treatments receiving 20–40 t ha –1 per single application showed smaller amplitudes in the soil nitrate-nitrogen levels in autumn than the treatments receiving 60 t ha –1. Nevertheless, nitrate-nitrogen levels (0–90 cm) in autumn were at maximum 49 kg ha –1 higher in these treatments than in the untreated control. Based on yield and soil nitrate-N contents, no justification was found to legislate a reduction in the amount of a single application to as low as 20 t ha –1.

Mulching with compost improves growth of blue spruce in Christmas tree plantations

Compost derived from source-separated organic household waste was used in a blue spruce (Picea pungens var. glauca) Christmas tree plantation as a mulch in tree rows in a 3–year trial. The amounts, which were applied at the start of the experiment, were 29 t ha –1 (treatment 1) and 58 t ha –1 (treatment 2; dry matter), corresponding to 3 and 6 combined annual compost applications, respectively. The compost mulch reduced the shoot growth of the 6-year-old blue spruce trees in the first year after compost application by 6% and 8% in treatment 1 and 2, respectively. In the second year, growth was enhanced for 8% and 5% in treatment 1 and 2, respectively. The growth enhancement was well proportioned and characterized by increases in both shoot growth and the number of lateral buds. In the third year, shoot growth was no longer significantly different from the untreated control. The soil contents of plant-available nitrogen and potassium, pH and electrical conductivity increased significantly through the compost application. The slightly increased water content of the soil positively influenced tree growth at this dry and sandy site.

Fate of Ah-receptor agonists in organic household waste during anaerobic degradation—estimation of levels using EROD induction in organ cultures of chick embryo livers

The fate of 7-ethoxyresorufin O-deethylase (EROD)-inducing compounds in source-separated organic household waste subject to anaerobic degradation (i.e. mesophilic/thermophilic anaerobic treatment) was investigated using organ cultures of embryonic chicken livers from fertilised hen eggs. This bioassay reflects the combined effect of all EROD-inducing, possibly dioxin-like compounds in a sample, including chemicals that are seldom or never analysed. All samples tested induced EROD in the bioassay, indicating the presence of dioxin-like compounds. In the anaerobic processes, the amounts of acid-resistant EROD-inducing compounds coming out of the reactors were considerably higher than the incoming amounts, especially for the low-temperature (mesophilic) process. This apparent production of EROD-inducing compounds may be due to de novo synthesis or to an increase in the EROD-inducing potency of the compounds in the material.

Reduction of fecal streptococcus and salmonella by selected treatment methods for sludge and organic waste

The increasing utilization of waste water sludge and source-separated organic household waste in agriculture has brought the quality aspects into focus, among others the hygienic aspects. In this study, the reducting effect on Fecal Streptococcus (FS) and Salmonella of different methods for stabilization and methods for further treatment of sludge and organic waste has been investigated. The most common methods for stabilization, i.e. aerobic and anaerobic stabilization, only reduce the indicator organisms by approximately 1 logarithmic decade. Methods for further treatment of sludge and organic waste have shown reductions of microorganisms allowing for unrestricted utilization in agriculture, meeting the product control: FS below 100/g and no Salmonella detected. The effect of storage of sludge at summer and winter temperature respectively has been investigated. At temperatures (around 20°C) significant reductions of Salmonella were found, while the die out at low temperatures (below 10°C) was limited. FS was not reduced systematically during storage, and therefore, FS is not usable as indicator organism for the hygienic properties of sludge during storage.

Calculating heavy metal accumulation in soil: a comparison of methods illustrated by a case-study on compost application

Accumulation of heavy metals in agricultural soil can be prevented by achieving an input that is smaller than, or equal to, the output. This is known as the ‘balance approach’. Heavy metal balance sheets can be used to determine the contribution of different input and output flows to the resulting accumulation. The change in heavy metal content in the plough layer is the result of the net difference between input and output flows per unit time. Existing balance approaches lack an analysis of the effect of soil amendments on soil composition. However, soil composition determines soil bulk density and plough layer weight and, hence, the resulting change in heavy metal concentration. It is, therefore, important to understand and account for the processes that affect soil composition. Recycling of compost produced from source-separated organic household waste as a soil amendment in agriculture is used as an example to illustrate how changes in soil composition complicate accumulation calculations. The dynamic soil composition balance approach is presented as a way to handle these complications by calculating mass balances of both heavy metals and main soil constituents. Comparing results of the dynamic soil composition balance with those of other balance approaches reveals the necessity to account for changes in soil composition and for the effect of soil-bound heavy metal flows in calculating heavy metal accumulation in agricultural soil. This new balance approach is of special relevance if organic soil amendments (e.g., manure and compost) and soil-bound heavy metal flows (e.g., erosion) are involved.

Reduction of fecal streptococcus and salmonella by selected treatment methods for sludge and organic waste

The increasing utilization of waste water sludge and source-separated organic household waste in agriculture has brought the quality aspects into focus, among others the hygienic aspects. In this study, the reducting effect on Fecal Streptococcus (FS) and Salmonella of different methods for stabilization and methods for further treatment of sludge and organic waste has been investigated. The most common methods for stabilization, i.e. aerobic and anaerobic stabilization, only reduce the indicator organisms by approximately I logarithmic decade. Methods for further treatment of sludge and organic waste have shown reductions of microorganisms allowing for unrestricted utilization in agriculture, meeting the product control: FS below 100/g and no Salmonella detected.The effect of storage of sludge at summer and winter temperatures respectively has been investigated. At temperatures (around 20'C) significant reductions of Salmonella were found, while the die out at low temperatures (below 10'C) was limited. FS was not reduced systematically during storage, and therefore, FS is not usable as indicator organism for the hygienic properties of sludge during storage.