The mesophilic and thermophilic anaerobic digestion of coffee waste containing coffee grounds

The anaerobic digestion of waste water containing significant levels of coffee grounds was assessed in mesophilic and thermophilic batch studies and CSTRs fed daily. A 58% reduction in VS was seen in both batch studies. Proximate compositional analysis showed that the waste had a high lipid component (26–33%). Levels of lipid, hemicellulose, α-cellulose and lignin were determined before and after digestion. These components were reduced as follows: lipid by 87% in the mesophilic study and 65% in the thermophilic study, α-cellulose by 51% in both mesophilic and thermophilic batch studies, hemicellulose by 22% in the mesophilic studies and 64% in the thermophilic studies. The lignin component was not reduced in either study. Mesophilic continuous digestion was achieved at a loading rate of 1.3 kg COD m⁻³ day⁻¹ (25 day HRT) for 99 days. Addition of sodium bicarbonate alone was not sufficient for long term anaerobic digestion. Addition of Ca(OH)₂, nitrogen, phosphorus and trace elements, however, gave successful digestion with COD and VS removal of 60% and a gas production rate of 0.34 11⁻¹ day⁻¹ (65–70% methane). Low levels of TVFA and high levels of bicarbonate alkalinity were present. Thermophilic digestion could be established at 1.6 kg COD m⁻³ day⁻¹ (20 day HRT) with the addition of sodium bicarbonate alone, or Ca(OH)₂ with nitrogen, phosphorus and trace elements. However long term digestion could not be established beyond 50 days without a increase in TVFA occurring.     

Anaerobic acidogenic digestion of olive mill wastewaters in biofilm reactors packed with ceramic filters or granular activated carbon

Four identically configured anaerobic packed bed biofilm reactors were developed and employed in the continuous acidogenic digestion of olive mill wastewaters to produce volatile fatty acids (VFAs), which can be exploited in the biotechnological production of polyhydroxyalkanoates. Ceramic porous cubes or granular activated carbon were used as biofilm supports. Aside packing material, the role of temperature and organic loading rate (OLR) on VFA production yield and mixture composition were also studied. The process was monitored through a chemical, microbiological and molecular biology integrated procedure. The highest wastewater acidification yield was achieved with the ceramic-based technology at 25 °C, with an inlet COD and an OLR of about 17 g/L and 13 g/L/day, respectively. Under these conditions, about the 66% of the influent COD (not including its VFA content) was converted into VFAs, whose final amount represented more than 82% of the influent COD. In particular, acetic, propionic and butyric acids were the main VFAs by composing the 55.7, 21.5 and 14.4%, respectively, of the whole VFA mixture. Importantly, the relative concentrations of acetate and propionate were affected by the OLR parameter. The nature of the packing material remarkable influenced the process performances, by greatly affecting the biofilm bacterial community structure. In particular, ceramic cubes favoured the immobilization of Firmicutes of the genera Bacillus, Paenibacillus and Clostridium, which were probably involved in the VFA producing process.     

Biological treatment at low temperatures of sulfur-rich phenols containing oil shale ash leachate

Aerobic and sequential anaerobic-aerobic treatment of oil shale ash dump leachate (COD 2000–3000 mg l⁻¹) was studied at 7–10° and at 20°C. The leachate was known to contain several phenolic compounds and have a high sulfur concentration. In the sequential anaerobic-aerobic processes, the chemical oxygen demand (COD) removals were most of the time slightly lower at 10°C (67%) than at 20°C (73%) while the biochemical oxygen demand for 7 days (BOD₇) removals were 97–99% at both temperatures. In the anaerobic stages, the COD removal was a result of sulfide production. In the single aerobic process at 20°C, the COD removal was 65% while at 7°C the COD removal was 54%. After the feed was changed from leachate to phenol in the 7°C aerobic reactor, the COD removal stabilized to about 95%. In all the leachate treatment processes studied, the total phenols removals were on average 78–86%. The anaerobic stages removed total phenols minimally. An aerobic process at 7–20°C alone seems feasible for treating oil shale ash leachate.     

Reducing the environmental footprint of wastewater screenings through anaerobic digestion with resource recovery

Screenings produced as the first stage of wastewater treatment and currently disposed of to landfill, are rich in volatile organic solids, nitrogen and phosphorus which could be recovered through anaerobic digestion. Biochemical methane potential (BMP) tests on screenings demonstrated a methane yield of 0.33 m³ methane/kg volatile solids (VS) and a VS destruction of 50%. Consequently, the effect of a range of hydraulic retention time (HRT) and organic loading rates (OLRs) was evaluated in lab‐scale continuously fed mesophilic digesters. The highest methane yield of 0.416 Nm³ methane/kg VS added was observed with an HRT of 15 days and an OLR of 2.5 kg VS/m³/day, when up to 65% of the VS were destroyed. If treated by anaerobic digestion, every dry tonne of screenings digested would divert 466 kg from landfill, save 4.6 tonne equivalent carbon dioxide (CO_{2 eq}) and deliver 3.4 MWh of renewable energy.     

Thermophilic anaerobic digestion of pulp and paper mill primary sludge and co-digestion of primary and secondary sludge

Anaerobic digestion of pulp and paper mill primary sludge and co-digestion of primary and secondary sludge were studied for the first time in semi-continuously fed continuously stirred tank reactors (CSTR) in thermophilic conditions. Additionally, in batch experiments, methane potentials of 210 and 230 m³CH₄/t volatile solids (VS)_added were obtained for primary, and 50 and 100 m³CH₄/tVS_added for secondary sludge at 35 °C and 55 °C, respectively. Anaerobic digestion of primary sludge was shown to be feasible with organic loading rates (OLR) of 1-1.4 kgVS/m³d and hydraulic retention times (HRT) of 16-32 d resulting in methane yields of 190-240 m³CH₄/tVS_fed. Also the highest tested OLR of 2 kgVS/m³d and the shortest HRT of 14-16 d could be feasible, if pH stability is confirmed. Co-digestion of primary and secondary sludge with an OLR of 1 kgVS/m³d and HRTs of 25-31 d resulted in methane yields of 150-170 m³CH₄/tVS_fed. In the digestion processes, cellulose and hemicellulose degraded while lignin did not. pH adjustment and nitrogen deficiency needs to be considered when planning anaerobic digestion of pulp and paper mill wastewater sludges.     

Degradation of phenol in wastewater in an upflow anaerobic sludge blanket reactor

Phenol in wastewater could be effectively degraded in an upflow anaerobic sludge blanket (UASB) reactor. With a 1:1 effluent recycle ratio, over 97% of phenol was removed at 37°C and pH 6.9-7.5 with 12 h of hydraulic retention time for phenol concentration up to 1260 mg·1⁻¹, corresponding to 3000 mg·1⁻¹ of chemical oxygen demand (COD) and a loading rate of 6 g-COD·1⁻¹·day⁻¹. The seed sludge took about 7 wk to develop the phenol-degrading capability which was sensitive to shocks. The bioactivity deteriorated readily when the granules were exposed to sudden changes of temperature and loading. Although the damage was not permanent, the recovery of bioactivity was gradual and lengthy. At 6 g-COD·1⁻¹·day⁻¹, each gram of granules was able to convert 0.49 g of COD into methane daily. On the average, about 94.7% of the total COD removed was converted to methane, while the rest was converted to biomass with a net yield of 0.038 g-VSS·(g-COD-removed)⁻¹. Electron micrographs show that the granules were composed of, among others, Syntrophus buswellii-, Methanothrix-, Methanospirillum- and Methanobrevibacter-like bacteria.     

Effect of temperature on the anaerobic thermophilic conversion of volatile fatty acids by dispersed and granular sludge

The effect of temperature on the rate of volatile fatty acid (VFA) conversion by thermophilic methanogenic sludge, cultivated at 55°C, was studied using both batch activity tests and continuous flow experiments. The temperature dependence of acetate conversion in the range between 37–70°C could be described by an Arrhenius derived model when dispersed sludge with a low specific activity was used. For this sludge the optimum acetate conversion rate was found at 65°C. However, the maximum acetate utilization rate was not affected by temperature in the range between 50°C to 65°C when granular sludge with a high specific methanogenic activity was used. Crushing the granules led to a 2 to 3 fold increase in the maximum activity at 60–65°C, indicating that the conversion rate was very likely limited by the diffusion rate of acetate into the granules. Similar results were obtained with butyrate as the substrate. The temperature dependence of the crushed granules was similar to that of the less active dispersed sludge. In contrast, the thermophilic propionate oxidation rate was highest with the intact granular sludge while a similar temperature dependence was found for both the granular and dispersed sludges. The affinity for VFA increased with decreasing temperature. This phenomenon was most pronounced for the granular sludge. The thermophilic treatment of a VFA-mixture in a UASB reactor appeared to be only slightly affected by temperature when moderately low loading rates were applied, i.e. 20 kg COD·m⁻³·d⁻¹. However, temperature had a strong effect applying loading rates of 40–90 kg COD·m⁻³·d⁻¹ accompanied with high effluent VFA concentrations. The results reveal a high thermostability of the thermophilic wastewater treatment process in the range 45–60°C if “high-rate” reactors with a granular sludge bed are used.     

Mitigating ammonia inhibition of thermophilic anaerobic treatment of digested piggery wastewater: use of pH reduction, zeolite, biomass and humic acid

High free ammonia released during anaerobic digestion of livestock wastes is widely known to inhibit methanogenic microorganisms and result in low methane production. This was encountered during our earlier thermophilic semi-continuously fed continuously-stirred tank reactor (CSTR) treatment of piggery wastewater. This study explored chemical and biological means to mitigate ammonia inhibition on thermophilic anaerobic treatment of piggery wastewater with the aim to increase organic volatile carbon reduction and methane production. A series of thermophilic anaerobic batch experiments were conducted on the digested piggery effluent to investigate the effects of pH reduction (pH 8.3 to 7.5, 7.0 and 6.5) and additions of biomass (10% v/v and 19% v/v anaerobic digested piggery biomass and aerobic-anaerobic digested municipal biomass), natural zeolite (10, 15 and 20 g/L) and humic acid (1, 5 and 10 g/L) on methane production at 55 °C for 9-11 days. Reduction of the wastewater pH from its initial pH of 8.3 to 6.5 produced the greatest stimulation of methane production (3.4 fold) coupled with reductions in free ammonia (38 fold) and total volatile fatty acids (58% TVFA), particularly acetate and propionate. Addition of 10-20 g/L zeolite to piggery wastewater with and without pH reduction to 6.5 further enhanced total VFA reduction and methane production over their respective controls, with 20 g/L zeolite producing the highest enhancement effect despite the ammonia-nitrogen concentrations of the treated wastewaters remaining high. Without pH reduction, zeolite concentration up to 20 g/L was required to achieve comparable methane enhancement as the pH-reduced wastewater at pH 6.5. Although biomass (10% v/v piggery and municipal wastes) and low humic acid (1 and 5 g/L) additions enhanced total VFA reduction and methane production, they elevated the residual effluent total COD concentrations over the control wastewaters (pH-unadjusted and pH-reduced) unlike zeolite treatment. The outcomes from these batch experiments support the use of pH reduction to 6.5 and zeolite treatment (10-20 g/L) as effective strategies to mitigate ammonia inhibition of the thermophilic anaerobic treatment of piggery wastewater.     

High-rate anaerobic treatment of Fischer-Tropsch wastewater in a packed-bed biofilm reactor

This study investigates the anaerobic treatment of an industrial wastewater from a Fischer-Tropsch (FT) process in a continuous-flow packed-bed biofilm reactor operated under mesophilic conditions (35 °C). The considered synthetic wastewater has an overall chemical oxygen demand (COD) concentration of around 28 g/L, mainly due to alcohols. A gradual increase of the organic load rate (OLR), from 3.4 gCOD/L/d up to 20 gCOD/L/d, was adopted in order to overcome potential inhibitory effects due to long-chain alcohols (>C6). At the highest applied OLR (i.e., 20 gCOD/L/d) and a hydraulic retention time of 1.4 d, the COD removal was 96% with nearly complete conversion of the removed COD into methane. By considering a potential of 200 tCOD/d to be treated, this would correspond to a net production of electric energy of about 8 × 10⁷ kWh/year.During stable reactor operation, a COD balance and batch tests showed that about 80% of the converted COD was directly metabolized through H₂⁻ and acetate-releasing reactions, which proceeded in close syntrophic cooperation with hydrogenotrophic and acetoclastic methanogenesis (contributing to about 33% and 54% of overall methane production, respectively). Finally, energetic considerations indicated that propionic acid oxidation was the metabolic conversion step most dependent on the syntrophic partnership of hydrogenotrophic methanogens and accordingly the most susceptible to variations of the applied OLR or toxicity effects.     

Performance of UASB reactor treating leachate from acidogenic fermenter in the two-phase anaerobic digestion of food waste

This study was conducted to investigate the performance of the upflow anaerobic sludge blanket (UASB) reactor treating leachate from acidogenic fermenter in the two-phase anaerobic digestion of food waste. The chemical oxygen demand (COD) removal efficiency was consistently over 96% up to the loading rates of 15.8 g COD/l d. The methane production rate increased to 5.51/l d. Of all the COD removed, 92% was converted to methane and the remaining presumably to biomass. At loading rates over 18.7 g COD/l d, the COD removal efficiency decreased due to sludge flotation and washout in the reactor, which resulted from short HRT of less than 10.6 h. The residual propionate concentration was the highest among the volatile fatty acids (VFA) in the effluent. The specific methanogenic activity (SMA) analysis showed that the VFA-degrading activity of granule was the highest for butyrate, and the lowest for propionate. Typical granules were found to be mainly composed of microcolonies of Methanosaeta. The size distribution of sludge particles indicated that partially granulated sludge could maintain the original structure of granular sludge and continue to gain size in the UASB reactor treating leachate from acidogenic fermenter.     

Operational window of a deammonifying sludge for mainstream application in a municipal wastewater treatment plant

The present work aimed to study the mainstream feasibility of the deammonifying sludge of side stream of municipal wastewater treatment plant (MWWTP) in Kaster, Germany. For this purpose, the deammonifying sludge available at the side stream was investigated for nitrogen (N) removal with respect to the operational factors temperature (15–30°C), pH value (6.0–8.0) and chemical oxygen demand (COD)/N ratio (≤1.5–6.0). The highest and lowest N-removal rates of 0.13 and 0.045 kg/(m³ d) are achieved at 30 and 15°C, respectively. Different conditions of pH and COD/N ratios in the SBRs of Partial nitritation/anammox (PN/A) significantly influenced both the metabolic processes and associated N-removal rates. The scientific insights gained from the current work signifies the possibility of mainstream PN/A at WWTPs. The current study forms a solid basis of operational window for the upcoming semi-technical trails to be conducted prior to the full-scale mainstream PN/A at WWTP Kaster and WWTPs globally.     

Removal of organics from leachates by anaerobic filter

A laboratory-scale study was conducted on two anaerobic filters at several loading rates and four hydraulic detention times. Feed substrates were landfill leachates taken from a recently opened landfill (Keele Valley) and from an older site (Brock North) which had been closed for about 8 years. The strong raw leachate from the new landfill had a COD of 14,000 mg l⁻¹, a BOD/COD ratio of 0.7 and a COD/P value of 17,900. The partially stabilized leachate from the older landfill had a COD of only 3750 mg l⁻¹, a BOD/COD ratio of 0.3 and a COD/P value of 30,640.Results from the treatment of the two leachates were compared with those from a previous study of a “mature” landfill (Beare Road). It was demonstrated that the anaerobic filter could reduce the COD of leachate from landfills of different ages by 90%, at loading rates of 1.26–1.45 kg COD m⁻³ d⁻¹. Total biogas production ranged between 400 and 500 l gas kg⁻¹ COD destroyed and methane content between 75 and 85%. No phosphorus addition was required over the loading range studied.     

Removal of micropollutants from urban wastewater using a UASB reactor coupled to a MBR at different organic loading rates

This study examines the removal of micropollutants (MPs) in a hybrid process that combines anaerobic and aerobic redox conditions under different Organic Loading Rates (OLRs).

A laboratory-scale pilot-plant composed of an Upflow Anaerobic Sludge Blanket reactor (UASB) combined with a membrane bioreactor (MBR) was operated. Six MPs were analyzed: the hormones estrone, 17-α-ethinyl estradiol and 17-β-estradiol, the plasticizer bisphenol A and the pharmaceuticals carbamazepine and diclofenac. In order to study its influence on removal efficiencies, the system was operated at three different OLRs: high (0.67 ± 0.15 kg COD/m³ d), medium (0.37 ± 0.06 kg COD/m³ d) and low (0.11 ± 0.02 kg COD/m³ d).

The results demonstrated the synergistic effects due to the double biological treatment, with removal rates above 90% for the hormones and the plasticizer. Pharmaceuticals were the most resistant compounds, being only partially removed in the liquid phase. Removal rates of the MPs were higher at high OLR of the influent.     

Performance analysis of a full-scale duckweed-covered sewage lagoon

A sewage lagoon for 2000–3000 capita (0.6 ha) has been operated successfully with a duckweed cover for over four years. The cover suppressed algal growth; the effluent turbidity was always below 12 Ntu. Because of inappropriate construction, one fifth of the inflow is lost by percolation and seepage during the dry season; during the wet season the loss is limited. During a detailed sampling period in the dry season actual hydraulic retention time was 20.4 d, and surface loading rate was 48–60 kgBOD₅/ha · d. Concentration reduction was 90–97% for COD, 95–99% for BOD₅, and 74–77% for Kjeldahl-N and total P. Effluent contained 2.7 mg Kjeldahl-N/l and 0.4 mg total P/l. The water column remained aerobic. At two-thirds of retention time the plants had absorbed virtually all NH⁺₄ and ortho-PO³⁻₄ from the water column. The duckweed harvest would remove in a watertight lagoon 60–80% of the N and P load, or 0.26 gN/m² · d and 0.05 gP/m² · d (in the first three-quarters of retention time). The results during this period were representative for the 4-year operation so far. Corrected for the leakage, plant productivity under these fertilised and managed conditions was sustained for several years at the level of 58–105 kg(dw)/ha · d, or 715–1200 kg/ha · d (over full lagoon surface) in the dry and wet season, respectively. We suggest that the microbial hydrolysis of the more complex organic N and P into NH⁺₄ and ortho-PO³⁻₄ is the limiting step for enhanced biomass production.     

Continuous treatment of the organic fraction of municipal solid waste in an anaerobic two-stage membrane process with liquid recycle

The stability and performance of a two-stage anaerobic membrane process was investigated at different organic loading rates (OLRs) and Hydraulic Retention Times (HRTs) over 200 days. The Hydrolytic Reactor (HR) was fed with the Organic Fraction of Municipal Solid Waste (OFMSW), while the leachate from the HR was fed continuously to two Submerged Anaerobic Membrane Bioreactors (SAMBR1 and 2). The Total COD (TCOD) of the leachate varied over a wide range, typically between 4000 and 26,000 mg/L while the Soluble COD (SCOD) in the permeate was in the range 400-600 mg/L, achieving a COD removal greater than 90% at a HRT of 1.6-2.3 days in SAMBR1. The operation was not sustainable below this HRT due to a membrane flux limitation at 0.5-0.8 L/m² h (LMH), which was linked to the increasing MLTSS. SCOD in the recycled permeate did not build up indicating a slow degradation of recalcitrants over time. SAMBR2 was run in parallel with SAMBR1 but its permeate was treated aerobically in an Aerobic Membrane Bioreactor (AMBR). The AMBR acted as a COD-polishing and ammonia removal step. About 26% of the recalcitrant SCOD from SAMBR2 could be aerobically degraded in the AMBR. In addition, 97.7 % of the ammonia-nitrogen was converted to nitrate in the AMBR at a maximum nitrogen-loading rate of 0.18 kg NH₄⁺-N/m³ day. GC-MS analysis was performed on the reactor effluents to determine their composition and what compounds were recalcitrant.     

Elutriated acid fermentation of municipal primary sludge

The performance of a novel fermentation process, adopting a sludge blanket type configuration, for higher hydrolysis/acidogenesis of the municipal primary sludge was investigated under batch and semi-continuous conditions with varying pH and temperature. This acid elutriation slurry reactor provided higher system performance with a short HRT (5d) and higher acidogenic effluent quality under pH 9 and thermophilic (55 degrees C) conditions. The hydrolysis of the sludge was revealed to be significantly dependent on seasonal effects for sludge characteristics but with little impact on acidogenesis. Based on the rainy season at the optimal conditions, VFA production and recovery fraction (VFA(COD)/COD) were 0.18 g VFA(COD)/g VSS(COD) and 63%. As byproducts, nitrogen and phosphorus release were measured at 0.006 g N/g VSS(COD) and 0.003 g P/g VSS(COD), respectively. For the mass balance in a full-scale plant (Q=158,880 m(3)/d) based on the rainy season, the VFA and non-VFA (as COD) production were 3110 kg VFA(COD)/d and 1800 kg COD/d, resulting in an increase of organics of 31 mg COD/L and 20mg VFA(COD)/L and nutrients of 0.7 mg N/L and 0.3 mg P/L in the influent sewage. The economical benefit from this process application was estimated to be about 67 dollars per 1000 m(3) of sewage except for energy requirements and also, better benefits can be expected during the dry season. Moreover, the results revealed that the process has various additional advantages such as pathogen-free stabilized solids production, excellent solids control and economical benefits.     

Research of parameters affecting sludge digestion process

The parameters characterizing and affecting anaerobic sludge digestion process in three Lithuanian cities are investigated. Results showed that digestion process was effective in all three objects: specific biogas production varied between 0.77 and 0.86 m³/kg volatile solids destroyed, methane content in the biogas varied between 64 and 67% and they complied with the values presented in the scientific references. The increase of VS loading (up to 2.6 kg VS/m³/day) led to the higher level of VS destruction which was up to 59%. The sludge retention time (hydraulic retention time) values must be in ranges of 20–25 days for the digestion process in order to get the design VS destruction, as well the biogas production. In a case of lack of raw materials, the application of wastes from food industry can help to reach the design parameters. The application of wastes from milk industry showed positive results in the digestion process.     

Universelle Energiekennzahlen für Deutschland – Teil 1: Differenzierte Kennzahlverteilungen nach Energieträger und wärmetechnischem Sanierungsstand

Universelle Energiekennzahlen für Deutschland bezeichnen eine innovative Datenbank, in der aktuell 1/4 Million Gebäude‐Energieverbrauchskennzahlen der BRUNATA‐METRONA‐Gruppe aus den vergangenen Jahren normiert zusammengefasst sind. Die Datenaufbereitung umfasst (1) eine rückwirkende Revision aller meteorologischen Klimakorrekturen mithilfe neuer ortsgenauer Klimafaktoren des Deutschen Wetterdienstes, (2) eine Teil mengenbildung von 18 Kennzahlvergleichsgruppen nach drei Hauptenergie‐ trägern und sechs wärmetechnischen Sanierungsständen sowie (3) eine kontinuierliche Abbildung der Kennzahlverteilungen als Funktion der Gebäudegröße. Die Mediane der Energiekennzahlverteilungen bewegen sich über alle Gebäudegrößen und Vergleichsgruppen etwa zwischen 80 und 175 kWh·m^–2·a^–1. Die absoluten Unterschiede der Mediane der Energiekennzahlverteilungen machen zwischen Neubauten und Gebäuden mit nominell wärmetechnisch unsanierter Gebäudehülle absolut zwischen 40 und 60 kWh·m^–2·a^–1 aus, relativ für alle Größenklassen und Energieträger generell weniger als 30 %. Beim direkten Vergleich zwischen wärmetechnisch sanierten und unsanierten Gebäuden sinkt dieser Relativanteil unter 20 % für gas‐ und ölbeheizte, auf etwa 25 % für fernwärmebeheizte Objekte. Andererseits betragen die statistischen Verteilungsbreiten der Kennzahlverteilungen selbst schon je ±σ = ±25 bis ±55 kWh·m^–2·a^–1. Die Energiekennzahl‐Mediane für Neubauten nach 1995 liegen für die Energieträger Gas und Öl über 100 kWh·m^–2·a^–1, für Fernwärmeobjekte über 80 kWh·m^–2·a^–1. Ein Ansatz zur Einschätzung möglicher Sanierungspotentiale in absoluten Energiebeträgen sowie eine Hochrechnung des wohnflächengewichteten und am aktuellen Klima orientierten Mittelwertes des spezifischen Energieverbrauchs für den nationalen Mietwohnungsbestand auf ca. 120 kWh·m^–2·a^–1 werden vorgestellt. Universal energy ratings for Germany – (Part 1): Differentiated rating distributions according to energy source and energy efficiency standard. Universal energy ratings for Germany represent an innovative database in which a quarter of a million energy consumption ratings for buildings recorded in past years by the BRUNATA METRONA Group are currently summarised in a standardised form. Data preparation includes (1) a retrospective review of all the meteorological corrections with the aid of new, localised climate factors from the German Weather Service, (2) a subset of 18 rating comparison groups according to three main energy sources and six levels of energy efficiency standard and (3) a continuous illustration of the rating distributions as a function of building size. The medians of the energy rating distributions range between about 80 and 175 kWh·m^–2·a^–1across all building sizes and comparison groups. Generally, the absolute differences between the medians of the energy rating distributions of new buildings and unrenovated, existing buildings are between 40 and 60 kWh·m^–2·a^–1. The relative differences for all sizes and energy sources are generally less than 30 %. If one compares renovated and unrenovated buildings directly, the relative figure drops to below 20 % for gas‐heated and oil‐heated and to about 25 % for buildings with district heating. On the other hand, the statistical distribution ranges of the rating distributions themselves are as wide as ±σ = ±25 to ±55 kWh·m^–2·a^–1 respectively. The energy rating medians for new buildings built after 1995 and using gas and oil as energy sources are above 100 kWh·m^–2·a^–1. They are above 80 kWh·m^–2·a^–1for buildings with district heating. An approach for gauging the possible renovation potential in terms of absolute energy and an extrapolation of the mean value of specific energy consumption for the country's existing rented apartments, weighted in terms of living area and based on the current climate, to approx. 120 kWh·m^–2·a^–1 are presented.     

Competition between planktonic and fixed microorganisms during the start-up of methanogenic biofilm reactors

The influence of the hydraulic retention time (HRT) on the start-up phase of a methanogenic inverse turbulent bed bioreactor was investigated. Two identical reactors were monitored, the only differing parameter being the HRT: one of the reactors was fed with a diluted wastewater at a constant HRT of 1 day, the organic loading rate (OLR) being increased by decreasing the substrate dilution; the second reactor was fed at a constant influent concentration of 20 g COD L(-1), the OLR being increased by decreasing the HRT from 40 days to 1 day. After 45 days of start-up, both reactors were operated at an OLR of 20 g COD L(-1)d(-1) and a HRT of 1 day. However, strong differences were observed on biofilm growth. In the reactor operated at a constant short HRT, biofilm concentration was 4.5 as high as in the reactor operated at an increasing HRT. This difference was attributed to the competition between planktonic and biofilm microorganisms in the latter reactor, whereas suspended biomass was quickly washed out in the former reactor because of the low HRT.     

Waste activated sludge fermentation: Effect of solids retention time and biomass concentration

Laboratory scale, room temperature, semi-continuous reactors were set-up to investigate the effect of solids retention time (SRT, equal to HRT hydraulic retention time) and biomass concentration on generation of volatile fatty acids (VFA) from the non-methanogenic fermentation of waste activated sludge (WAS) originating from an enhanced biological phosphorus removal process. It was found that VFA yields increased with SRT. At the longest SRT (10 d), improved biomass degradation resulted in the highest soluble to total COD ratio and the highest VFA yield from the influent COD (0.14 g VFA-COD/g TCOD). It was also observed that under the same SRT, VFA yields increased when the biomass concentration decreased. At a 10 d SRT the VFA yield increased by 46%, when the biomass concentration decreased from 13 g/L to 4.8 g/L. Relatively high nutrient release was observed during fermentation. The average phosphorus release was 17.3 mg PO₄-P/g TCOD and nitrogen release was 25.8 mg NH₄-N/g TCOD.