Impact of organic loading rate and reactor design on thermophilic anaerobic digestion of mixed supermarket waste

A mixture of supermarket food waste from bakery, butchery, cooked meats and cheese, fishmonger, fruit, and vegetable sections was subjected to anaerobic digestion under thermophilic conditions (55 °C). Lab-scale induced bed reactors (IBR) and completely stirred tank reactors (CSTR) were operated at different organic loading rates (OLR), i.e., 3.0, 3.6 and 4.6 kg volatile solids (VS) per m³ of reactor and day. Regardless of the type of reactor, an OLR of 3.6 kg VS/m³·day was found to be the optimum, achieving up to 48.1% more methane production per kg of treated waste than for the other OLRs tested. In general, there were no statistically significant differences (p-value < 0.05) between IBR and CSTR performance at the same OLR tested. However, for the optimum OLR, the IBR achieved a mean methane production of 1.5 L CH₄/L_reactor·day (426.7 L CH₄/kg VS) and the highest VS removal (89.0%, on average). This reactor obtained 22.1% more CH₄ yield than the analogous CSTR and the highest methane content in the biogas (66.9% CH₄). Finally, the process was successfully tested under large-scale conditions (1.25 m³ IBR pilot-plant). The CH₄ production and the biodegradation yield were in line with those obtained in the lab-scale IBR.

Inverted phase fermentation as a pretreatment for anaerobic digestion of cattle manure and sewage sludge

The aim of this research study was to analyse the effect of applying inverted phase fermentation (IPF) prior to the anaerobic digestion of cattle manure and sewage sludge. IPF promotes the endogenous bacteria present in waste and hence enzymatic hydrolysis, producing a solid-liquid separation. The clarified bottom layer or liquid phase (LP, 70% volume in manure and 65% in sludge), and the thickened top layer or solid phase (SP, 30% volume in manure and 35% in sludge) were digested separately. Operating at 37 °C, the time needed to digest the LP from manure was shorter (10 days) than that needed to digest the corresponding SP or the untreated substrate (22 days in both cases). The time needed to digest the separated phases of sludge (LP: 2 days, SP: 15 days) was lower than that needed to digest manure. Biogas production rates for the manure after pretreatment were 0.5 L/L·day for the SP and 0.7 L/L·day for the LP, allowing higher OLR (4.5-4.8 gCOD/L·day) than when digesting untreated manure and increasing biogas production by 17%. IPF applied to sewage sludge led to a production of 1.8 L/L·day at an OLR of 6.2 gCOD/L·day for the SP and 2.0 L/L·day at 12.9 gCOD/L·day for the LP. Assuming a conventional OLR of 2-3 gCOD/L·day, the advantage of applying IPF to sewage sludge resides in the possibility of operating digesters at much higher OLR.

Impact of secondary inorganic aerosol and road traffic at a suburban air quality monitoring station

PM10 from a suburban site in the northwest of Spain was assessed using data from chemical determinations, meteorological parameters, aerosol maps and five-day back trajectories of air masses. Temporal variations in the chemical composition of PM10 were subsequently related to stationary/mobile local sources and long-range transport stemming from Europe and North Africa. The presence of secondary inorganic species (sulphates, nitrates and ammonium) in airborne particulate matter constituted one of the main focuses of this study. These chemical species formed 16.5% of PM10 on average, in line with other suburban background sites in Europe. However, a maximum of 47.8% of PM10 were recorded after several days under the influence of European air masses. Furthermore, the highest values of these three chemical species coincided with episodes of poor air circulation and influxes of air masses from Europe. The relationship between SO₄^2- and NH₄⁺ (R² = 0.57, p-value<0.01) was found to improve considerably in summer and spring (R² = 0.88 and R² = 0.87, respectively, p-value<0.01), whereas NO₃^- and NH₄⁺ (R² = 0.55, p-value<0.01) reproduced this pattern in winter (R² = 0.91, p-value<0.01). The application of a receptor model to the dataset led to the identification of notable apportionments due to road traffic and other types of combustion processes. In fact, large amounts of particulate matter were released to the atmosphere during episodes of biomass burning in forest fires. On isolated days, combustion was estimated to contribute up to 21.0 μg PM/m³ (50.8% of PM10). The contribution from industrial processes to this source is also worth highlighting given the presence of Ni and Co in its profile. Furthermore, African dust outbreaks at the sampling site, characterised by an arc through the Atlantic Ocean, were usually associated with a higher concentration of Al₂O₃ in PM10. Results evidenced the relevance of stationary (i.e., steelworks and thermal power station) and mobile sources in the air quality at the suburban site under study, with important apportionments of particulate matter coming from road traffic and as consequence of releasing precursor gases of secondary particles to the atmosphere.

Traffic tracers in a suburban location in northern Spain: relationship between carbonaceous fraction and metals

PM10 and black smoke were monitored at a suburban sampling station located in the northern Spanish city of Gijón. Thirty-two metals and total carbon (TC) (i.e., organic carbon (OC) and elemental carbon (EC)) were analyzed over a year. The study of air-mass origin based on 5-day back trajectories was carried out to assess its influence on the recovery data. Different strategies were implemented to infer the influence of traffic in the area. On average, TC accounted for 29 % of the PM10 fraction, with OC forming 77 % of this TC. The influence of traffic was clearly reduced during intense Atlantic advection episodes, when OC and EC decreased up to 0.39 and 0.22 μg C/m(3), respectively. In contrast, the highest values were reported during regional episodes, exceeding 10 μg C/m(3) of OC and 2 μg C/m(3) of EC. The correlation between EC and OC was found to notably improve when considering the days with high traffic flow (from R (2) = 0.46 to R (2) = 0.74). This pattern was also reproduced by black smoke and EC (from R (2) = 0.49 to R (2) = 0.59). Cu and Sn were found to be reliable traffic tracers given their high dependence on EC (R (2) = 0.82 and R (2) = 0.79, respectively). Nevertheless, Sn, Ba, and Sb showed a better correlation with Cu than EC, suggesting a common origin. In the case of Sn, R (2) improved from 0.79 to 0.91. The Cu/Sb ratio had a mean value of 6.6 which agrees with diagnostic criterions for brake wear particles. The relationships and ratios between EC, Cu, Sb, Sn, Ba, and Bi pointed out to non-exhaust emissions, playing a significant role in the chemical composition of PM10. Brake wear was presented as the most likely origin for Cu, Sb, and Sn.

Differences in soluble COD and ammonium when applying inverted phase fermentation to primary, secondary and mixed sludge

Primary, secondary and mixed sludge were treated by inverted phase fermentation. This treatment results in solid-liquid separation of sludge after endogenous enzymatic hydrolysis (anaerobic conditions: 42°C, 48 hours). The soluble chemical oxygen demand (sCOD) was increased in the solid phase up to 1,800%, 21,300% and 260% in primary, secondary and mixed sludge, respectively. The corresponding increase in sCOD in the liquid phase accordingly reached values of up to 440%, 5,100% and 140%. Phase separation led to an enrichment of volatile solids in the solid phase (89-358% primary sludge, 80-102% secondary sludge and 29-133% mixed sludge). The NH4+-N values increased notably after the endogenous enzymatic hydrolysis itself. To investigate the short-term evolution following the treatment, the variation in sCOD, NH4+-N and solids was also monitored after keeping the hydrolysate at 37°C under anaerobic conditions for 24 hours. This stage showed no generalized pattern in terms of sCOD.

Differences in soluble COD and ammonium when applying ultrasound to primary, secondary and mixed sludge

Ultrasound treatment is often applied to enhance the anaerobic digestion of sludge. Optimal conditions for organic matter solubilisation of primary, secondary and mixed sludge were assessed by implementing ultrasound disruption at different specific energies (from 3,500 to 21,000 kJ/kgTS). The variation in soluble chemical oxygen demand (sCOD) and ammonium nitrogen (NH4+-N) was monitored following the treatment, and after a subsequent fermentation (24 h, 37 °C). The effect of the treatment was clearly more pronounced in secondary sludge than in the other types of sludge. Relatively minimal values in solubility were found when applying ultrasound at different energies depending on the sludge (3,500-7,000 kJ/kgTS in primary sludge and 10,500-14,000 kJ/kgTS in secondary sludge). This minimal value was not so noticeable in mixed sludge. The addition of inoculum was not required after ultrasound disruption in order to perform the subsequent fermentation. After this final stage, no general pattern in terms of sCOD was observed. Increases and decreases were conditioned by the coverage of the ultrasound irradiation; NH4+-N values increased notably during the fermentation.

Effect of ultrasound pre-treatment in the anaerobic co-digestion of cattle manure with food waste and sludge

This paper presents a study of the effect of applying ultrasound pre-treatment in the production of methane when co-digesting mixtures of cattle manure with food waste and sludge. A series of experiments were carried out under mesophilic and thermophilic conditions in continuously stirred-tank reactors containing 70% cattle manure, 20% food waste and 10% sewage sludge. Ultrasound pre-treatment allows operating at lower HRT, achieving higher volumetric methane yields: 0.85 L CH4/L day at 36°C and 0.82 CH4/L day at 55°C, when cattle manure and sewage sludge were sonicated. With respect to the non-sonicated waste, these values represent increases of up to 31% and 67% for mesophilic and thermophilic digestion, respectively.

Thermophilic co-digestion of cattle manure and food waste supplemented with crude glycerin in induced bed reactor (IBR)

The aim of the present research work was to boost biogas production from cattle manure (CM) by adding food waste (FW) and crude glycerin (Gly) from the biodiesel industry as co-substrates. For this purpose, different quantities of FW and Gly were added to CM and co-digested in an induced bed reactor (IBR) at 55 °C. Sonication pre-treatment was implemented in the CM+Gly mixture, applying 550 kJ/kg TS to enhance the biodegradability of these co-substrates. The best results were obtained with mixtures of 87/10/3 (CM/FW/Gly) (w/w) operating at an organic loading rate of 7 g COD/L day, obtaining 92% COD removal, a specific methane yield of 640 L CH4/kg VS and a methane production rate of 2.6L CH4/L day. These results doubled those obtained in the co-digestion of CM and FW without the addition of Gly (330 L CH4/kg VS and 1.2L CH4/L day).

Methane production from cattle manure supplemented with crude glycerin from the biodiesel industry in CSTR and IBR

The aim of the present research work was to optimise biogas production from cattle manure by adding crude glycerin from the biodiesel industry. For this purpose, 6%v/v crude glycerin (the optimum amount according to previous research) was added to ground manure and the mixture was sonicated to enhance biodegradability prior to anaerobic co-digestion at 55 °C. Two different reactors were used: continuously stirred (CSTR) and induced bed (IBR). The methanol and pure glycerin contents of the crude glycerin used in this study were 5.6% and 49.4% (w/w), respectively. The best results when operating in CSTR were obtained for an organic loading rate (OLR) of 5.4 kg COD/m(3) day, obtaining 53.2m(3) biogas/t wet waste and 80.7% COD removal. When operating in IBR, the best results were obtained for an OLR of 6.44 kg COD/m(3)day, obtaining 89.6% COD removal and a biogas production of 56.5m(3)/t wet waste.

Co-digestion of cattle manure with food waste and sludge to increase biogas production

Anaerobic co-digestion strategies are needed to enhance biogas production, especially when treating certain residues such as cattle/pig manure. This paper presents a study of co-digestion of cattle manure with food waste and sewage sludge. With the aim of maximising biogas yields, a series of experiments were carried out under mesophilic and thermophilic conditions using continuously stirred-tank reactors, operating at different hydraulic residence times. Pretreatment with ultrasound was also applied to compare the results with those obtained with non-pretreated waste. Specific methane production decreases when increasing the OLR and decreasing HRT. The maximum value obtained was 603 LCH(4)/kg VS(feed) for the co-digestion of a mixture of 70% manure, 20% food waste and 10% sewage sludge (total solid concentration around 4%) at 36°C, for an OLR of 1.2g VS/L day. Increasing the OLR to 1.5g VS/L day led to a decrease of around 20-28% in SMP. Lower methane yields were obtained when operating at 55°C. The increase in methane production when applying ultrasound to the feed mixtures does not compensate for the energy spent in this pretreatment.

Optimization of biogas production from cattle manure by pre-treatment with ultrasound and co-digestion with crude glycerin

Biogas production by co-digestion of cattle manure with crude glycerin obtained from biodiesel production was studied after pre-treatment of the cattle manure or mixtures of cattle manure with different amounts of added glycerin with ultrasound. Batch experiments with 1,750 mL of medium containing 1,760 g of screened cattle manure or mixtures of cattle manure (screened or ground) and 70-140 mL or crude glycerin were incubated under mesophilic and thermophilic condition in stirred tank reactors. Under mesophilic conditions, the addition of 4% glycerin to screened manure increased biogas production by up to 400%. Application of sonication (20 kHz, 0.1 kW, and 4 min) to a mixture of manure+4% glycerin increased production of biogas by up to 800% compared to untreated manure. The best results were obtained under thermophilic conditions using sonicated mixtures of ground cattle manure with 6% added glycerin (348 L methane/kg COD removed were obtained).

Reducing the environmental impact of methane emissions from dairy farms by anaerobic digestion of cattle waste

Four dairy cattle farms considered representative of Northern Spain milk production were studied. Cattle waste was characterised and energy consumption in the farms was inventoried. Methane emissions due to slurry/manure management and fuel consumption on the farms were calculated. The possibility of applying anaerobic digestion to the slurry to minimise emissions and of using the biogas produced to replace fossil fuels on the farm was considered. Methane emissions due to slurry management (storage and use as fertiliser) ranged from 34 to 66kg CH(4)cow(-1)year(-1) for dairy cows and from 13 to 25kg CH(4)cow(-1)year(-1) for suckler calves. Cattle on these farms are housed for most of the year, and the contribution from emissions from manure dropped in pastures is insignificant due to the very low methane conversion factors. If anaerobic digestion were implemented on the farms, the potential GHG emissions savings per livestock unit would range from 978 to 1776kg CO(2)eq year(-1), with the main savings due to avoided methane emissions during slurry management. The methane produced would be sufficient to supply digester heating needs (35-55% of the total methane produced) and on-farm fuel energy requirements.

Colour, turbidity and COD removal from old landfill leachate by coagulation-flocculation treatment

The application of different coagulants and flocculants to leachate from an old landfill to determine the optimum conditions for removal of organic matter, colour and turbidity is studied. Ferric chloride, aluminium sulphate, aluminium polychloride (PAX) and polyacrylamide polyelectrolytes were tested. Higher pollutant removals (73% COD, 98% colour and 100% turbidity) were obtained using ferric chloride at pH 5.0-5.5 and for a dosage of 0.6 g Fe l(- 1). The volume of sludge generated after centrifugation represents about 4.0-4.6% when ferric chloride or aluminium sulphate is used, and 10% when employing aluminium polychloride. When flocculants were also added, the results obtained were similar to those found when adding only coagulants, although a considerable increase in the settling rate was obtained.

Physico-chemical analysis and calorific values of poultry manure

Spain is one of the major producers of broilers and laying hens in the European Union, with an overall market share of around 12%. The poultry manure that is produced is usually employed as fertilizer on cropland, either directly or after a composting process. In some cases, however, this waste is transported over 120km to be used as fertilizer, with the resulting high transport costs. In other countries, poultry manure is used as an alternative natural fuel source for power generation. In this study, poultry manure from all the laying hen farms in Asturias was characterized with a view to its possible use as an energy source. The Higher Heating Values on a dry basis (experimental) varies between 12,052 and 13,882kJ/kg. Lower Heating Values (LHVs) on a wet basis range are much lower (mean values of 2664kJ/kg) due to the high moisture content of poultry manure. Accordingly, the co-combustion of this waste with other types of waste such as forest and wood waste (LHV on a wet basis of 8044 and 15,830kJ/kg, respectively) or municipal solid waste (LHV on a wet basis of 10,725kJ/kg) should be considered as an alternative energy source. Chlorine and sulphur contents in dry matter vary around mean values of 0.64% and 0.11%, respectively. The waste also presents high amounts of Ca (4.84%) and K (2.38%).

Physico-chemical and biological treatment of MSW landfill leachate

This paper analyses the evolution of the physico-chemical characteristics of the leachate from the Central Landfill of Asturias (Spain), which has been operating since 1986, as well as different treatment options. The organic pollutant load of the leachate, expressed as chemical oxygen demand (COD), reached maximum values during the first year of operation of the landfill (around 80,000 mg/L), gradually decreasing over subsequent years to less than 5000 mg/L. The concentration of ammonium, however, has not decreased, presenting values of up to 2000 mg/L. When feasible, recirculation can greatly decrease the organic matter content of the leachate to values of 1500-1600 mg COD/L. Applying anaerobic treatment to leachates with a COD between 11,000 and 16,000 mg/L, removal efficiencies of 80-88% were obtained for organic loading rates of 7 kg COD/m3d. For leachates with lower COD (4000-6000 mg/L), the efficiency decreased to around 60% for organic loading rates of 1 kg COD/m3d. Applying coagulation-flocculation with iron trichloride or with aluminium polychloride, it was possible to reduce the non-biodegradable organic matter by 73-62% when treating old landfill leachate (COD: 4800 mg/L, BOD5: 670 mg/L), also reducing turbidity and colour by more than 97%. It is likewise possible to reduce the non-biodegradable organic matter that remains after biological treatment by adsorption with activated carbon, although adsorption capacities are usually low (from 15 to 150 mg COD/g adsorbent). As regards ammonium nitrogen, this can be reduced to final effluent values of 5 mg/L by means of nitrification/denitrification and to values of 126 mg/L by stripping at pH 12 and 48 h of stirring.

Denitrification of high nitrate concentration wastewater using alternative carbon sources

The use of different organic carbon sources in the denitrification of wastewater containing 2500 mg nitrates/L in a SBR was studied. Three alternative sources of carbon were tested: wastewater from a sweet factory, a residue from a soft drinks factory and a residue from a dairy plant. The first two are sugar-rich, whereas the third presents a high content in lactic acid. Maximum specific denitrification rates of between 42 and 48 mg NO(3)-N/g VSS h were obtained. The effluents were nitrate-free and very low COD concentrations were obtained in 4-6h reaction time, especially with the sugar-rich carbon sources. The values of the denitrifier net yield coefficient were higher than when using methanol (0.93-1.75 g VSS(formed)/g NO(x)-N(reduced)). The lowest value was obtained using the lactic acid-rich residue. The optimum COD/N ratios varied between 4.6 for the lactic acid-rich carbon source and 5.5-6.5 for the sugar-rich carbon sources.

Anoxic-aerobic treatment of the liquid fraction of cattle manure

Cattle manure from a dairy farm was treated in order to reduce its pollution potential. The manure came from a farm with 120 cows kept in stables in a free stall barn. As pretreatment, the manure is usually filtered on the farm using a screw press separator with a 0.5 mm mesh. Approximately 70% of the total filtered volume passes through the screen, thus constituting the liquid fraction. This fraction, with a composition of around 64,500 mg COD/l, 5770 mg total-N/l and 800 mg total-P/l, was subjected to centrifugation followed by a two-step biological treatment (anoxic-aerobic) to reduce organic matter (COD), nitrogen and phosphorus compounds. Centrifugation led to the following removal efficiencies: 35% total solids, 60% COD, 75% total phosphorus and 20% total nitrogen (mainly organic nitrogen). With the subsequent anoxic-aerobic treatment, average removal efficiencies of 85% for COD, 90% for total phosphorus and 75% for total nitrogen were achieved.

Three-step biological process for the treatment of the liquid fraction of cattle manure

The liquid fraction of cattle manure was subjected to a biological treatment combining anoxic-anaerobic and oxic processes in order to stabilize the organic matter and reduce nitrogen and phosphorus so as to avoid problems of pollution when applying it to the land. The anoxic process was carried out at 30 degrees C in a CSTR reactor, the anaerobic process in a UASB reactor at 37 degrees C and the oxic treatment in another CSTR at 20 degrees C. The following results were obtained when working under optimum conditions (removal efficiencies in brackets): COD was reduced from 42 to 3.8 g/L (>90%); total solids from 41 to 14 (67%); total volatile solids from 22 to 7.0 (68%); total Kjeldahl nitrogen from 2.2 to 0.1 g/L (95%); NH4(-)-N from 1.10 to 0.02 g/L (98%) and Total-Phosphorus from 0.696 to 0.058 g/L (92%). Nitrates, undetected in the liquid fraction of cattle manure, were present in the final effluent as a result of nitrification. To reduce the amount of nitrates, different recirculation rates were tested. The minimum nitrate concentration achieved was 127 mg/L using a recirculation ratio of 4.

Denitrification of wastewater containing high nitrate and calcium concentrations

The removal of nitrate from rinse wastewater generated in the stainless steel manufacturing process by denitrification in a sequential batch reactor (SBR) was studied. Two different inocula from wastewater treatment plants were tested. The use of an inoculum previously acclimated to high nitrate concentrations led to complete denitrification in 6h (denitrification rate: 22.8mg NO3- -N/gVSSh), using methanol as carbon source for a COD/N ratio of 4 and for a content of calcium in the wastewater of 150mg/L. Higher calcium concentrations led to a decrease in the biomass growth rate and in the denitrification rate. The optimum COD/N ratio was found to be 3.4, achieving 98% nitrate removal in 7h at a maximum rate of 30.4mg NO3- -N/gVSSh and very low residual COD in the effluent.

Coagulation-flocculation as a pretreatment process at a landfill leachate nitrification-denitrification plant

The main aim of this research work was to study the possible application of coagulation-flocculation as a pretreatment process for young landfill leachate in order to prevent fouling in the ultrafiltration membranes employed for the separation of biomass in the biological plant. Jar-test experiments were carried out to determine the optimum conditions for the removal of turbidity colour and organic matter. The coagulants ferric chloride, aluminium sulphate and aluminium polychloride (PAX) were tested, along with different types of flocculants (anionic and cationic polyelectrolytes). Optimum pH values were around 4.0 and 6.0 for ferric chloride and aluminium sulphate, respectively. It was not necessary to alter the pH of the leachate when using PAX, as the optimum value was found to be similar to that of the leachate (around 8.3). Optimum dosages were 0.4 g Fe(3+)/L, 0.8 g Al(3+)/L and 4 g PAX/L, although there was not much difference in the results for lower dosage of PAX. The best results were found with this coagulant, obtaining 98% turbidity removal, 91% colour removal and 26% COD removal. When flocculants were also added, the results were similar to those found when adding only coagulants, although a considerable increase in the settling rate was obtained. The volume of the sludge generated represents around 4.5-5.0% when using ferric chloride or aluminium sulphate, and 15% when using aluminium polychloride.