Long-term phosphorus removal by Ca and Fe-rich drainage filter materials under variable flow and inlet concentrations.

Phosphorus (P) losses from tile-drained agricultural fields may degrade surface water quality by accelerating eutrophication. Among the different edge-of-field technologies, compact filter systems using different filter materials have been identified as potentially effective solutions for removing P from drainage water before discharge downstream. This study investigated the long-term (>696 days) P removal efficiency of 5 different filter materials in a column setup, using artificial drainage water (pH 6). Filter materials included two iron-based granulates (calcinated diatomaceous earth (CDE), ferric hydroxide granules (CFH)), and three calcium-based granulates (seashells, limestone, calcinated silicate/calcium oxide (Filtralite-P)). Experiments were performed under variable flow rates (0.037 and 1.52 L h-1; hydraulic retention time of 26-43 min and 18-30 h) and inlet P concentrations (0.14 and 0.7 mg L-1). An overall analysis revealed that the Fe-based materials achieved higher P retention than Ca-based materials. In particular, CFH was capable of retaining 99 and 98 % of the high and low inlet P concentrations, respectively. Conversely, limestone retained only 25 % of the high P load. CDE performed moderately well, independently of the inlet P concentration. Filtralite-P and Seashells performed well at high inlet P concentration but relatively poorly at low P concentration. The sensitivity of filter material P removal efficiency to variations in P loading was generally lowest for CFH and highest for limestone.

Combinations of fungal and milling pretreatments for enhancing rice straw biogas production during solid-state anaerobic digestion.

Rice straw was pretreated by different combinations of physical (milling) and biological (incubation with Pleurotus ostreatus fungus) treatment to improve its biodegradability and biogas production during solid-state anaerobic digestion (SS-AD). Effects of milling (⩽2mm) and incubation time (10, 20 and 30d), on lignin, cellulose, and hemicellulose degradation during fungal pretreatment and methane yield during digestion were assessed by comparison with untreated rice straw. Both incubation time and milling had significant impacts on both lignin removal during fungal pre-treatment and methane yield during digestion. A combination of fungal pretreatment at 30days followed by milling prior to anaerobic digestion resulted in 30.4% lignin removal, the highest selectivity value (the ratio between relative lignin removal and relative cellulose removal) of 4.22, and the highest methane yield of 258L/kgVS. This was equivalent to a 165% increase in methane yield from SS-AD compared to untreated rice straw.

Improvement in CH4/CO2 ratio and CH4 yield as related to biomass mix composition during anaerobic co-digestion.

Sixteen data sets (two of which were measured in this study) with a combined total of 145 measurements of ultimate methane yield (UMY) during mono- and co-digestion of ternary biomass mixtures were used to assess impact of co-digestion on the relative change in UMY (ΔUMY) as a function of biomass mix composition. The data involved 9 biomass materials (brewery spent grains, chicken manure, cow manure, fresh grass clippings, pig manure, primary sewage sludge, vegetable food waste, wheat straw, and rice straw). Results of the assessment shows that co-digestion in 85% of yields positive values of ΔUMY regardless of the biomass materials used, however, a smaller fraction (15%) resulted in negative ΔUMY during co-digestion. The data further indicate that for each set of ternary biomass material mixtures there exists an optimal biomass mix composition at which ΔUMY is at a maximum. Statistical analyses based on the data used here indicate that the maximum value of ΔUMY (ΔUMYmax) is always positive regardless of biomass materials being co-digested.

Uncertainty in degradation rates for organic micro-pollutants during full-scale sewage sludge composting.

Composting can potentially remove organic pollutants in sewage sludge. When estimating pollutant removal efficiency, knowledge of estimate uncertainty is important for understanding estimate reliability. In this study the uncertainty (coefficient of variation, CV) in pollutant degradation rate (K1) and relative concentration at 35days of composting (C35/C0) was evaluated. This was done based on recently presented pollutant concentration data, measured under full-scale composting conditions using two different sampling methods for a range of organic pollutants commonly found in sewage sludge. Non-parametric statistical procedures were used to estimate CV values for K1 and C35/C0 for individual pollutants. These were then used to compare the two sampling methods with respect to CV and to determine confidence intervals for average CV. Results showed that sampling method is crucial for reducing uncertainty. The results further indicated that it is possible to achieve CV values for both K1 and C35/C0 of about 15%.

Improving biogas quality and methane yield via co-digestion of agricultural and urban biomass wastes.

Impact of co-digestion versus mono-digestion on biogas and CH4 yield for a set of five biomass materials (vegetable food waste, cow dung, pig manure, grass clippings, and chicken manure) was investigated considering 95 different biomass mixes of the five materials under thermophilic conditions in bench-scale batch experiments over a period of 65days. Average biogas and CH4 yields were significantly higher during co-digestion than during mono-digestion of the same materials. This improvement was most significant for co-digestion experiments involving three biomass types, although it was independent of the specific biomasses being co-digested. Improvement in CH4 production was further more prominent early in the digestion process during co-digestion compared to mono-digestion. Co-digestion also appeared to increase the ultimate CH4/CO2 ratio of the gas produced compared to mono-digestion although this tendency was relatively weak and not statistically significant.

Biogas and methane yield in response to co- and separate digestion of biomass wastes.

The impact of co-digestion as opposed to separate digestion, on biogas and methane yield (apparent synergetic effects) was investigated for three biomass materials (pig manure, cow manure and food waste) under mesophilic conditions over a 36 day period. In addition to the three biomass materials (digested separately), 13 biomass mixtures (co-digested) were used. Two approaches for modelling biogas and methane yield during co-digestion, based on volatile solids concentration and ultimate gas and methane potentials, were evaluated. The dependency of apparent synergetic effects on digestion time and biomass mixture composition was further assessed using measured cumulative biogas and methane yields and specific biogas and methane generation rates. Results indicated that it is possible, based on known volatile solids concentration and ultimate biogas or methane yields for a set of biomass materials digested separately, to accurately estimate gas yields for biomass mixtures made from these materials using calibrated models. For the biomass materials considered here, modelling indicated that the addition of pig manure is the main cause of synergetic effects. Co-digestion generally resulted in improved ultimate biogas and methane yields compared to separate digestion. Biogas and methane production was furthermore significantly higher early (0-7 days) and to some degree also late (above 20 days) in the digestion process during co-digestion.

Quantifying measurement uncertainty in full-scale compost piles using organic micro-pollutant concentrations.

Reductions in measurement uncertainty for organic micro-pollutant concentrations in full scale compost piles using comprehensive sampling and allowing equilibration time before sampling were quantified. Results showed that both application of a comprehensive sampling procedure (involving sample crushing) and allowing one week of equilibration time before sampling reduces measurement uncertainty by about 50%. Results further showed that for measurements carried out on samples collected using a comprehensive procedure, measurement uncertainty was associated exclusively with the analytic methods applied. Application of statistical analyses confirmed that these results were significant at the 95% confidence level. Overall implications of these results are (1) that it is possible to eliminate uncertainty associated with material inhomogeneity and (2) that in order to reduce uncertainty, sampling procedure is very important early in the composting process but less so later in the process.

Cities as development drivers: from waste problems to energy recovery and climate change mitigation.

There is a strong connection between economic growth and development of cities. Economic growth tends to stimulate city growth, and city economies have often shaped innovative environments that in turn support economic growth. Simultaneously, social and environmental problems related to city growth can be serious threats to the realization of the socio-economic contributions that cities can make. However, as a result of considerable diversity of competences combined with interactive learning and innovation, cities may also solve these problems. The 'urban order' may form a platform for innovative problem solving and potential spill-over effects, which may stimulate further economic growth and development. This paper discusses how waste problems of cities can be transformed to become part of new, more sustainable solutions. Two cases are explored: Aalborg in Denmark and Malmö in Sweden. It is shown that the cities have the potential to significantly contribute to a more sustainable development through increased material recycling and energy recovery. Waste prevention may increase this potential. For example, instead of constituting 3% of the total greenhouse gas emission problem, it seems possible for modern European cities to contribute to greenhouse gas emission reduction by 15% through up to date technology and integrated waste management systems for material and energy recovery. Going from being part of the problem to providing solutions; however, is not an easy endeavour. It requires political will and leadership, supportive regulatory frameworks, realistic timetables/roadmaps, and a diverse set of stakeholders that can provide the right creative and innovative mix to make it possible.

Energy and greenhouse gas balances for a solid waste incineration plant: a case study.

Energy and greenhouse gas balances for a waste incineration plant (Reno-Nord I/S, Aalborg, Denmark) as a function of time over a 45-year period beginning 1960 are presented. The quantity of energy recovered from the waste increased over time due to increasing waste production, increasing lower heating value of the waste and implementation of improved energy recovery technology at the incineration plant. Greenhouse gas (GHG) balances indicated progressively increasing GHG savings during the time period investigated as a result of the increasing energy production. The GHG balances show that the Reno-Nord incineration plant has changed from a net annual GHG emission of 30 kg CO(2)-eq person(-1) year(-1) to a net annual GHG saving of 770 kg CO(2)-eq person(-1) year(-1) which is equivalent to approximately 8% of the annual emission of GHG from an average Danish person (including emissions from industry and transport). The CO(2) emissions associated with combustion of the fossil carbon contained in the waste accounted for about two-thirds of the GHG turnover when no energy recovery is applied but its contribution reduces to between 10 and 15% when energy recovery is implemented. The reason being that energy recovery is associated with a large CO(2) saving (negative emission).

Optimizing feed composition for improved methane yield during anaerobic digestion of cow manure based waste mixtures.

This study investigated methane yield via anaerobic digestion of multi-component substrates based on mixtures of biodegradable single-component substrates with cow dung as main component. Bench and full-scale digestion experiments were carried out for both single and multi-component substrates to identify the relationship between methane yield and substrate composition. Results from both bench- and full-scale experiments corresponded well and showed that using multi-component substrates increases the methane yield much more than what would be expected from digestion of single substrates. Process stability as indicated by gas production, pH and NH(4)(+) concentration variations were also improved by using multi-component substrates compared to digestion of single-component substrates. The results, thus, suggest that assessment of methane yield for multi-component substrates cannot reliably be based on methane yields for corresponding single-component substrates but should instead be measured directly.

Gas dispersion and immobile gas volume in solid and porous particle biofilter materials at low air flow velocities.

Gas-phase dispersion in granular biofilter materials with a wide range of particle sizes was investigated using atmospheric air and nitrogen as tracer gases. Two types of materials were used: (1) light extended clay aggregates (LECA), consisting of highly porous particles, and (2) gravel, consisting of solid particles. LECA is a commercial material that is used for insulation, as a soil conditioner, and as a carrier material in biofilters for air cleaning. These two materials were selected to have approximately the same particle shape. Column gas transport experiments were conducted for both materials using different mean particle diameters, different particle size ranges, and different gas flow velocities. Measured breakthrough curves were modeled using the advection-dispersion equation modified for mass transfer between mobile and immobile gas phases. The results showed that gas dispersivity increased with increasing mean particle diameter for LECA but was independent of mean particle diameter for gravel. Gas dispersivity also increased with increasing particle size range for both media. Dispersivities in LECA were generally higher than for gravel. The mobile gas content in both materials increased with increasing gas flow velocity but it did not show any strong dependency on mean particle diameter or particle size range. The relative fraction of mobile gas compared with total porosity was highest for gravel and lowest for LECA likely because of its high internal porosity.

Organic micropollutant degradation in sewage sludge during composting under thermophilic conditions.

Degradation of 12 common organic micropollutants in sewage sludge representing bactericides, flame retardants, fragrances, vulcanizers, and plasticizers (part of many common products) during thermophilic composting was investigated. Micropollutant concentrations, compost temperature, water content, and organic matter content were measured over 24 days in a full-scale compost windrow made from digested sewage sludge, yard waste, and horse manure. Composting took place indoors, and the windrow was turned several times during the experimental period. Concentrations of all 12 micropollutants decreased during composting, and degradation was statistically significant for 7 of the 12 micropollutants. Metabolites (galaxolidone and methyl-triclosan) were produced from two micropollutants (galaxolide and triclosan) during composting, indicating microbial degradation. Pollutant concentrations early in the experiment were more variable than those experienced for the chemical method development. This was likely due to compost heterogeneity. After the second compost turning, concentrations became more stable as compost became more homogeneous.

Variability of soil potential for biodegradation of petroleum hydrocarbons in a heterogeneous subsurface.

Quantifying the spatial variability of factors affecting natural attenuation of hydrocarbons in the unsaturated zone is important to (i) performing a reliable risk assessment and (ii) evaluating the possibility for bioremediation of petroleum-polluted sites. Most studies to date have focused on the shallow unsaturated zone. Based on a data set comprising analysis of about 100 soil samples taken in a 16 m-deep unsaturated zone polluted with volatile petroleum compounds, we statistically and geostatistically analysed values of essential soil properties. The subsurface of the site was highly layered, resulting in an accumulation of pollution within coarse sandy lenses. Air-filled porosity, readily available phosphorous, and the first-order rate constant (k(1)) of benzene obtained from slurry biodegradation experiments were found to depend on geologic sample characterization (P<0.05), while inorganic nitrogen was homogenously distributed across the soil stratigraphy. Semivariogram analysis showed a spatial continuity of 4-8.6 m in the vertical direction, while it was 2-5 times greater in the horizontal direction. Values of k(1) displayed strong spatial autocorrelation. Even so, the soil potential for biodegradation was highly variable, which from autoregressive state-space modeling was partly explained by changes in soil air-filled porosity and gravimetric water content. The results suggest considering biological heterogeneity when evaluating the fate of contaminants in the subsurface.

Phosphorus and zinc dissolution from thermally gasified piggery waste ash using sulphuric acid.

Ash from thermally gasified piggery waste (GA) was treated with sulphuric acid (H(2)SO(4)) using two extraction methods. First different loads (0.39-0.98 kg H(2)SO(4)/kg ash) and concentrations (0.2-2M) were used in 3h extraction. Second, titration of 1:25 (w/w) ash:water suspension was conducted with 4M H(2)SO(4) to determine ash buffer capacity at nine pH steps from 12 to 0.1. Total P and zinc (Zn) dissolution was monitored. Optimal acid load and concentration to dissolve 94% P and 55% Zn from GA was 0.98 kg H(2)SO(4)/kg ash and 0.6M, respectively, which corresponds to acid demand of 19.2 kg H(2)SO(4)/kg P recovered. High concentrations (2M) did not improve P dissolution, but Zn was easier released. Ash buffer capacity was the highest at pH 4 and 0.1, first one due to dissolution of Ca, the second one due to autoprotolysis of water. Acid load had stronger effect on dissolution than concentration in the first method, however in the second; both factors had comparable effect.

Assessing the impacts of changes in treatment technology on energy and greenhouse gas balances for organic waste and wastewater treatment using historical data.

Historical data on organic waste and wastewater treatment during the period of 1970-2020 were used to assess the impact of treatment on energy and greenhouse gas (GHG) balances. The assessment included the waste fractions: Sewage sludge, food waste, yard waste and other organic waste (paper, plastic, etc.). Data were collected from Aalborg, a municipality located in Northern Denmark. During the period from 1970-2005, Aalborg Municipality has changed its waste treatment strategy from landfilling of all wastes toward composting of yard waste and incineration with combined heat and power production from the remaining organic municipal waste. Wastewater treatment has changed from direct discharge of untreated wastewater to full organic matter and nutrient (N, P) removal combined with anaerobic digestion of the sludge for biogas production with power and heat generation. These changes in treatment technology have resulted in the waste and wastewater treatment systems in Aalborg progressing from being net consumers of energy and net emitters of GHG, to becoming net producers of energy and net savers of GHG emissions (due to substitution of fossil fuels elsewhere). If it is assumed that the organic waste quantity and composition is the same in 1970 and 2005, the technology change over this time period has resulted in a progression from a net annual GHG emission of 200 kg CO( 2)-eq. capita(-1) in 1970 to a net saving of 170 kg CO(2)-eq. capita(-1) in 2005 for management of urban organic wastes.

Gaseous oxygen uptake in porous media at different moisture contents and airflow velocities.

The presence and distribution of water in the pore space is a critical factor for flow and transport of gases through unsaturated porous media. The water content also affects the biological activity necessary for treatment of polluted gas streams in biofilters. In this research, microbial activity and quantity of inactive volume in a porous medium as a function of moisture content and gas flow rate were investigated. Yard waste compost was used as a test medium, and oxygen uptake rate measurements were used to quantify microbial activity and effective active compost volume using batch and column flow-through systems. Compost water contents were varied from air-dry to field capacity and gas flows ranged from 0.2 to 2 L x min(-1). The results showed that overall microbial activity and the relative fraction of active compost medium volume increased with airflow velocity for all levels of water content up to a certain flow rate above which the oxygen uptake rate assumed a constant value independent of gas flow. The actual value of the maximum oxygen uptake rate was controlled by the water content. The oxygen uptake rate also increased with increasing water content and reached a maximum between 42 and 48% volumetric water content, above which it decreased, again likely because of formation of inactive zones in the compost medium. Overall, maximum possible oxygen uptake rate as a function of gas flow rate across all water contents and gas flows could be approximated by a linear expression. The relative fraction of active volume also increased with gas flow rate and reached approximately 80% for the highest gas flows used.

Gas phase dispersion in compost as a function of different water contents and air flow rates.

Gas phase dispersion in a natural porous medium (yard waste compost) was investigated as a function of gas flow velocity and compost volumetric water content using oxygen and nitrogen as tracer gases. The compost was chosen because it has a very wide water content range and because it represents a wide range of porous media, including soils and biofilter media. Column breakthrough curves for oxygen and nitrogen were measured at relatively low pore gas velocities, corresponding to those observed in for instance soil vapor extraction systems or biofilters for air cleaning at biogas plants or composting facilities. Total gas mechanical dispersion-molecular diffusion coefficients were fitted from the breakthrough curves using a one-dimensional numerical solution to the advection-dispersion equation and used to determine gas dispersivities at different volumetric gas contents. The results showed that gas mechanical dispersion dominated over molecular diffusion with mechanical dispersion for all water contents and pore gas velocities investigated. Importance of mechanical dispersion increased with increasing pore gas velocity and compost water content. The results further showed that gas dispersivity was relatively constant at high values of compost gas-filled porosity but increased with decreasing gas-filled porosity at lower values of gas-filled porosity. Results finally showed that measurement uncertainty in gas dispersivity is generally highest at low values of pore gas velocity.