Investigation of the Physical, Chemical and...

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1 Investigation of the Physical, Chemical and Microbiological Parameters Influencing the Small-scale In-vessel Composting of Food Waste D. Orthodoxou 1,b , T.R. Pettitt 2,a , M. Fuller 2 , M. Newton 2 , N. Knight 2 , S.R. Smith 1 1 Department of Civil and Environmental Engineering, Skempton Building, Imperial College London, South Kensington Campus, London SW7 2AZ, UK 2 Eden Project, Bodelva, Cornwall PL24 2SG, UK a Present address: National Pollen and Aerobiology Research Unit, University of Worcester, Worcester WR2 6AJ, UK b Present address: ISOTECH Ltd Environmental Research and Consultancy, 1 Kalliopis Street, 2102 Aglantzia, Nicosia, Cyprus Presenting author email: [email protected] Presenting author telephone number: +357 99437367

Transcript of Investigation of the Physical, Chemical and...

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Investigation of the Physical, Chemical and Microbiological Parameters

Influencing the Small-scale In-vessel Composting of Food Waste

D. Orthodoxou 1,b, T.R. Pettitt2,a, M. Fuller2, M. Newton2, N. Knight2, S.R. Smith1

1 Department of Civil and Environmental Engineering, Skempton Building, Imperial College London, South Kensington Campus, London SW7 2AZ, UK

2Eden Project, Bodelva, Cornwall PL24 2SG, UK a Present address: National Pollen and Aerobiology Research Unit, University of

Worcester, Worcester WR2 6AJ, UK b Present address: ISOTECH Ltd Environmental Research and Consultancy, 1 Kalliopis

Street, 2102 Aglantzia, Nicosia, Cyprus

Presenting author email: [email protected] Presenting author telephone number: +357 99437367

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Abstract Purpose: The principal disposal route for food waste is landfill where it decomposes anaerobically producing methane (CH4). Landfill space is however rapidly being depleted and strict regulations are limiting, and increasing the cost of, disposal of biodegradable waste to landfill. The composting of food waste provides an opportunity for local authorities and small to medium enterprises (SMEs) to reduce their waste disposal costs whilst increasing recycling rates. However, due to the readily putrescible nature and high moisture content of food waste, the composting process often operates sub-optimally. Methods: This paper investigates the interactions of physical, chemical and microbial parameters within an in-vessel food waste composting reactor while conditions are sub-optimal and after normal composting conditions become re-established. Results: A microbial response to moisture content was the main factor increasing the pH of the compost mixture and allowing thermophilic temperatures to develop. Interstitial gas analysis showed that anaerobic conditions in the reactor were not responsible for the decrease in pH, but that this response was explained by a combination of excessively wet and acidic feedstock. The monitoring of carbon dioxide (CO2) evolution and oxygen (O2) utilisation provided a good indication of the progress of the composting process. Conclusions: Moisture contents between 41 – 48 % were associated with a marked transitional increase in compost pH above 6.0 and a concomitant rise in temperature to thermophilic values. Moisture contents >48 % caused severe acidogenesis and mesophilic temperatures (<45 ºC). Under the conditions of this investigation, the upper critical moisture content (MC) for in-vessel composting of FW was 40 %. Keywords: waste management, food waste, composting, SME 1. Introduction Approximately 15 million tonnes of food waste (FW) are discarded annually in the UK by households and from commercial and industrial sources [1]. The principal disposal route for food waste is landfill, where it decomposes anaerobically producing methane (CH4), giving the equivalent of 0.45 t CO2 equivalents per t [2]. Additionally, landfill space is rapidly depleting and is expected to run out in the UK within 10 years [4] and restrictions imposed by the European Landfill Directive [5] are limiting, and increasing the cost of, disposal of biodegradable waste to landfill. The decentralised composting of food waste provides a real opportunity for local authorities and small to medium enterprises (SMEs) to reduce their waste disposal costs whilst increasing recycling rates. However, strict controls apply to the treatment of FW containing or potentially containing animal by-products [6]. European Commission Regulation No 142/2011 [7] requires the treatment of such FW within an enclosed environment, such as an in-vessel process, and that specified time-temperature conditions are met to fully sanitise the material.

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Composting relies on the activity of micro-organisms present in the waste stream to increase the temperature of the mixture to the thermophilic range (>45ºC) to stabilise and sanitise the treated product [8]. Optimum conditions in the compost feedstock are key to the success of the process and these are in the ranges: moisture content (MC): 40-60%, pH: 6-8, carbon/nitrogen (C/N) ratio: 25:1 to 40:1. The process also requires sufficient aeration and acceptable material structure with a bulk density of 500-700 kg m-3 [9]. Food waste alone does not have a suitable composition for in-vessel composting without the addition of bulking agents, as it tends to become highly homogeneous in size especially if pre-shredded, has a high moisture content (MC) and is often very acidic [10]. Furthermore, the required collection of FW in closed containers may establish anaerobic conditions further lowering its pH value below optimal levels [11]. These factors make FW a potentially challenging feedstock for composting.

In experiments with small-scale reactors, an initial decrease in the pH value is often observed after food waste addition to the in-vessel composter [12-14]. In most severe cases this can lead to a significant decline in process temperature, causing degradation to slow down and promoting the formation of malodours [13]. Volatile fatty acids (VFAs) in the compost mixture play a key role in regulating the pH value during the mesophilic phase [14, 15]. Acidified compost material (pH<6) contains undissociated VFAs, which are toxic to, and inactivate, aerobic micro-organisms, resulting in the accumulation of VFAs and the further acidification of the compost mixture. However, when the pH value of the mixture increases above pH 6.0, the dissociated, easily decomposable, forms of fatty acids predominate and aerobic organisms remain active [16].

Various management techniques have been used with varying degrees of success to lower the concentration of VFAs in the mixture and raise the compost pH and temperature, such as the: addition of acidophilic organisms [17], addition of mature compost [11, 13], increase of the feedstock protein content [14] and increase of the aeration rate [15]. These have been met with varying degrees of success. Furthermore, mesophilic aerobes are more acid tolerant than thermophilic organisms [18]. Therefore, maintaining the compost temperature below 46ºC until the pH value increases above pH 6.5, allows aerobic organisms to degrade VFAs [19], thus shortening the mesophilic phase and increasing the composting temperature in a shorter period of time.

Although some understanding of the processes involved in the composting of FW has been achieved, the majority of studies have been conducted using laboratory-scale reactors with highly controlled feedstocks to allow the identification of individual process parameters. However, the outcomes of laboratory-scale research do not always apply to larger, industrial-sized composting reactors where issues of scale and heterogeneity of feedstocks may affect the composting process.

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This paper presents the results of a detailed assessment of FW composting in a commercially operated, rotary in-vessel process that was performing sub-optimally, over a period of eight weeks. The research is not a controlled experiment in a strict sense, but provides data for an intensively monitored, operational system and is valuable because it relates to the actual performance of a representative FW composting process in practice. The aim of the research was to quantify the main physico-chemical factors influencing composting of FW in the in-vessel rotating drum reactor including temperature, pH, MC and interstitial gas concentrations, and to determine the influence of feedstock input regime and process control strategy on composting activity.

2. Methods

2.1. The study site The Eden Project, Bodelva, Cornwall (http://www.edenproject.com), attracts approximately one million visitors per year. Depending on visitor numbers, between 100 and 700 kg of FW are produced each day by the on-site catering facilities. An in-vessel rotary drum composter (Neter 30, Susteco) was installed to recycle the FW as compost for use in the various horticultural activities on-site. Therefore, Eden represents a good example of a medium-scale enterprise that is focussed on reducing food waste disposal by adopting on-site composting and recycling. However, due to the readily putrescible nature and high MC of FW, the composter was found to work sub-optimally on various occasions with acidic conditions and low temperatures prevailing for much of the composting mass.

2.2. In-vessel composter The composting system installed at the Eden Project was a Neter 30 (SustecoAB) in-vessel composter, which is a horizontal, stainless steel cylinder approximately 8m long and 2.1m in diameter (Fig. 1), with an internal volume of 30 m3 and a maximum design capacity of 730 t per annum. The drum was housed in a stainless steel outer-casing (approximately 9.5 m long, 2.6 m high and 2.7 m wide), to insulate the compost as well as to protect personnel from the rotating mechanism of the reactor. A 15 mm insulating layer enveloped the entire cylinder and provided additional insulation [20]. The outer-casing was fitted with four doors that align with four inner hatches, permitting inspection and sampling of the compost material. Material in the vessel travelled towards the discharge end through the action of new material entering at the feed end, by mixed plug-flow operation. The retention time within the reactor depended on the rate of feeding and varied between 60 and 110 days. Green waste was fed into the vessel through an auger via a hopper, whereas FW entered through a shredder set to reduce the particle size to 20 mm.

The compost inside the reactor was aerated by the turning action of the reactor. A fan also aerated the compost by negative pressure inside the vessel. The fan drew ambient

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air into the front of the outer-casing, and below the vessel to the discharge end, where it entered the vessel and travelled counter-current towards the feed end, passing over the composting material. Extracted air passed through a cyclonic filter to remove fine particulates, and was further treated by bio-filtration.

At the time of this investigation the composter was programmed to rotate for 6 minutes per hour. A full turn of the vessel was completed in two hours.

Fig. 1 Schematic representation of the Neter 30 in-vessel rotating drum composter. The sampling points are denoted by

2.3. Feedstock

The feedstock input into the vessel consisted of FW collected from the various catering facilities at the Eden Project site and shredded green waste (GW) from the horticultural activities on site. Additionally, shredded corrugated board dust (CBD), sawdust (SD) and sawdust pellets (PEL) were brought on site and used in the composter. Finally, finished compost (FC) discharged from the composter was reintroduced to the front end of the vessel as part of the feedstock regime. Table 1 shows the total amounts of each feedstock material supplied to the vessel per week. Over the period of the investigation, a total of 4,567 kg of feedstock material were loaded into the vessel and 2,094 kg of FC were discharged from the composter. Prior to the monitoring period, the loading rates of FW and bulking agents were suboptimal compared to recommended feedstock input requirements [9] and the composting process exhibited severe acidification with the pH value decreasing to 4.0.

Food waste collected on site at Eden had an average pH value of 5.0, which decreased further to pH 4.0 - 4.4 after shredding, presumably due to accelerated degradation processes following particle size reduction and the associated release of VFAs. Due to its high MC, shredded FW was dense with little porosity. The porosity of the compost mixture was increased by the addition of PEL, which expanded upon re-hydration and increased the free air space (FAS). The low MC of PEL (approximately 15%) also

Feedstock In

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Inspectio n Doors

8 m 2.1 m

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increased the dry solids (DS) content of the feedstock mixture. However the pH value of PEL was low at 4.7, and thus contributed to compost acidification. The pH value of non-pelletised SD was less acidic compared to PEL and was in the range 5.6 - 5.8. Shredded CBD was used as a drying-agent, due to its low MC, equivalent to approximately 11%, and for pH adjustment (pH value of 8.0). Shredded GW arising from various horticultural activities on site was used to increase the C/N ratio of the compost mixture, raise the pH value and to provide microbial populations suitable for composting. The exact characteristics of the GW varied, but in general it was a mixture of woody and leafy material with pH values ranging between 6.0 and 8.0.

Finished compost removed from the vessel was alkaline with pH values ranging between 7.5 and 8.5, was in the thermophilic temperature range and was significantly drier than the feed input material. Therefore, re-introducing FC to the feed end of the vessel was also applied as a strategy to raise the pH, reduce the MC of the feed input, and to supply a thermophilic population of microorganisms.

During the third week of sampling (16th–23rd June), 565 kg of material was removed from the discharge end of the vessel and re-introduced to the feed end, in an attempt to reduce the MC and raise the temperature of the compost mixture through better pH balance (Table 1). When this transfer of material failed to yield results, a new management strategy was implemented during weeks 4 to 7 of the monitoring period (24th June to 18th July). During this phase a large quantity of material from the discharge end of the vessel was removed, at a rate of approximately 335 kg per week, and the composter was fed with fresh shredded GW, FC and CBD or SD pellets. The GW, FC and CBD were introduced to the vessel in the approximate proportions of 50, 35 and 15 %, respectively. When PEL was substituted for CBD the ratio of GW, FC and SD changed to 54, 37 and 10 %, respectively. Food waste, equivalent to 572 kg, was supplied to the composter in week 1 of the monitoring period, but inputs of FW were suspended until week 6 when the rate of addition was equivalent to 14, 224 and 279 kg in week 6, 7 and 8, respectively. Table 1 Type and weight (kg) of feedstock material supplied to the in-vessel composter and the total amount (kg) of composted product discharged

Week 1 2 3 4 5 6 7 8 Input: FW

572

-

-

-

-

14

224

279

GW 208 300 101 197 84 350 205 84 FC 80 172 681(565) - - - 115 47 CBD 99 125 34 132 119 - - - SD - 5 22 - - - - -

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PEL - - - 16 - 242 30 30 Total Input: 959 602 838 345 203 606 574 440 Total Output:

194 -

(565)*

388

179

579

189

-

* Value in brackets represents the material removed from the discharge end of the vessel and recycled to the feed end.

2.4. Sampling and Experimental Techniques

Samples for analysis were taken four times a week, at ten positions inside the vessel. Two samples were taken from each of the hatch doors, approximately 50 cm from each other to make a total of eight samples, and two more samples were collected, one from the feed and one from the discharge end of the vessel. Sampling was performed using a trowel to take material from a depth of 10cm from the surface of the composting material, approximately 30 cm away from the doors. Approximately 1.5 L of material were collected from each sampling point in labelled 4 L plastic containers and the samples were promptly transferred to the laboratory for physico-chemical analysis. The temperature at each sampling point was measured using a digital thermometer (Minitemp Monitor, Martin Lishman, UK) attached to a 1.2 m probe.

Wet bulk density and MC were determined using standard techniques [21]. The pH of the samples was determined by adding 150 ml of deionised water to 30 ml of sample (5:1 ratio), stirring vigorously for one minute and followed by an equilibration period of one hour at room temperature [13].

Interstitial gas concentrations were monitored using a portable gas analyser (GA 2000 Portable Gas Analyser, Geotechnical Instruments, UK) attached to a 1 m sampling probe. The gases monitored were oxygen (O2 %), carbon dioxide (CO2 %), CH4 (%), and carbon monoxide (CO ppm).

3. Results

3.1. Baseline data and effects of management interventions Baseline data were collected during the first two weeks of the monitoring period. This showed that the MC of the compost mixture was approximately 60 %, the pH value was in the range pH 4.0 - 5.0 and the temperature was generally in the range of 30 - 35 oC at Hatch 1 - 3 (Fig. 2). However, the baseline data recorded a significant difference in activity between Hatch 3 and 4 during this period. This showed that the temperature and pH increased and MC declined during the latter stages of the drum composting process owing to effects of unspecified, preceding management conditions and the mixed, plug-flow operation, slow-lag response and long retention time of the drum composter. This behaviour is discussed further below. Hatch 4 data also showed that the conditions were changing with time and the MC approximately doubled over the baseline period from 23 % to 47 %.

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Recycling FC back into the process in week 3 influenced the properties of the composting mixture at Hatch 1 and reduced the MC to 45 – 50 % and increased the pH value and temperature to 5.0 – 5.5 and 42 – 45 oC, respectively (Fig. 2). However, there was a lag period of approximately a week after the second intervention (in week 4, 24th June) before a significant rise in temperature, above the mesophilic range, and an increase in pH value was recorded at Hatch 1. The impacts of the interventions on the composting process were detected at Hatch 2 during the latter stages of the monitoring period in week 7, but were more difficult to discern at Hatch 3, owing to the mixed, plug-flow operation of the reactor. However, these results suggested the establishment of effective composting activity and FW was reintroduced at increasing loading rates during week 6 to 8 (Table 1). A drop in pH value from approximately 9.0 to 6.3 on the final sampling event at Hatch 1 emphasised that ongoing assessment of the FW loading rate and pH status of the composting mixture was critical to prevent reacidification of the process.

3.2. Temperature, moisture content and pH Moisture content, pH and temperature are critical control parameters in composting operations and have a high degree of interdependency. Therefore, the interrelationships between, and the impacts and critical thresholds of these important parameters on the process were examined using pooled data from the monitoring programme.

The overall relationship between MC and temperature was tested by linear regression analysis and represented using a broken stick model (Fig. 3a); the linear relationship between these parameters was highly statistically significant (P<0.001) at moisture contents ≥37.7 %. Thus, compost temperature decreased in linear relation to increasing moisture contents above this value, but was not significantly related (P>0.05) to MC in dryer conditions. The effect of pH value on composting temperature followed an approximately curvilinear pattern (Fig. 3b). Maximum, thermophilic temperatures were measured above pH 6.5, but the temperature decreased with declining pH and the inhibition of thermophilic microbial activity was observed below approximately pH 6.5 [11].

The relationship between MC and pH was examined using pooled data for all sampling points and times and revealed three distinct regions (Fig. 4). The effect of MC on pH was strongly transitional in the range 41 - 48 %, corresponding with the transition from upper mesophilic to thermophilic conditions (Fig 3a). Below this range of moisture contents, the pH value was relatively consistent and alkaline in the approximate range pH 8.5 - 9.5. However, the pH value declined markedly at moisture contents >41 % and decreased below 5.5 in approximate linear relation with moisture contents >48 %.

Increased pH values were partly related to the input of more alkaline material with higher solids content (Fig. 2), however, the results also showed there was a physiological interaction between compost pH and MC. Thus, at Hatch 1, pH increased

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with decreasing MC due to inputs of alkaline bulking agents of higher DS compared to the composting material previously loaded into the drum (Fig. 5). However, the increase in pH at moisture contents <45 % was greater than the pH value of the input material. The pH also rose in response to decreasing moisture contents at Hatch 2 and to a smaller extent at Hatch 3, showing that mixing of bulking agents had occurred in these regions of the drum compared to the baseline data before the interventions.

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Fig. 2 Changes in temperature, % moisture content and pH over the eight-week sampling period

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However, the MC and pH value at Hatch 2 and 3 remained above and below the respective optimal thresholds of these parameters measured here (45 % and 6.5, respectively) compared to the alkaline pH conditions observed at Hatch 1.

Fig. 3 The effect of % moisture content (A) and pH (B) on the temperature of the

compost mixture

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% Moisture Content

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Fig. 4 The effect of % moisture content on pH The pH value at Hatch 4 showed a similar pattern of response to decreasing moisture as observed at Hatch 1; the critical MC was approximately 45 – 48 % and, below this range, compost pH value was neutral to alkaline whereas the compost acidified with increasing MC. In this case, however, the response was presumably associated with evaporative drying of material in the drum, since there was minimal influence and transfer of the new feedstock intervention regime at Hatch 4 over the timescale of the monitoring period compared to the overall retention time of the process. Nevertheless, the results demonstrated an equivalent microbiological response to decreasing MC and the improved physico-chemical environment, which increased the process pH presumably due to more efficient VFA metabolism, in both regions of the drum (Hatch 1 and 4). 3.3. Interstitial gas concentrations The concentrations of O2, CO2 and CO were recorded at each hatch at a depth of 60 cm (Fig. 6). The decomposition of organic matter releases heat, which causes the temperature in the compost mix to rise. When decomposition occurs aerobically CO2 is released. The increase in heat is then proportional to the amount of CO2 evolved and the amount of O2 utilised [13]. Therefore, the production of CO2 provides a measure of the degradation rate of the compost [18]. The concentration of CO2 at Hatch 1 increased significantly at the beginning of July when the temperature transitioned from mesophilic to thermophilic ranges. This was approximately one week after the removal

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of large amounts of acidified material from the vessel and the addition of fresh material to the front of the process.

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Fig. 5 Effect of % moisture content on pH at each hatch before and after the removal of acidified material and addition of fresh feedstock.

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Fig. 6 Changes in O2, CO2 and CO concentrations at hatches 1 and 5, and O2, CO2 and temperature at hatches 2 and 3 at 60cm into the compost mixture.

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The change also occurred at Hatch 2, but later in the sampling period, in response to the plug-flow and mixing of material in the rotary vessel. Carbon dioxide remained elevated whereas O2 concentrations declined (particularly at Hatch 1). When composting activity was high and temperatures were in the thermophilic range (Hatch 1), there was O2 depletion, and O2 values were very close to, or at, 0. As the temperature of the compost mixture increased and O2 became depleted, elevated concentrations of CO were recorded (Hatch 1 in July and Hatch 4 at the beginning of June). Further investigation is necessary to understand the mechanism and significance of CO release from compost, as well as its potential implications to occupational health and safety.

The evolution of CO2 and utilisation of O2 were inversely related at all the sampling points. The concentrations of both of gases were in linear relation to compost temperature (Fig. 7). As was also observed elsewhere [11-14], with increasing temperatures, the CO2 concentrations increased whereas O2 concentrations decreased.

Methane concentrations ranged between 0.1 - 2.0 % throughout the sampling period and did not appear to be affected by changes in temperature or to follow the evolution pattern of any other gases. 4. Discussion Feedback mechanisms have a major influence on the composting process, for instance, declining pH values can lead to further acidification of the compost matrix [13]. Under acidifying conditions, for example, resulting from the production of VFAs during rapid decomposition of FW, the temperature of the compost mixture generally fails to rise above the mesophilic range. Thus, the results reported here were consistent with earlier work [13] and showed that process temperatures did not increase above 45 ºC unless the pH value of the composting mixture was >pH 6.0 (Fig. 2).

Day et al. [12] showed that the composting of a mixture of FW and GW followed a gradual decrease in MC for the first three to four weeks followed by rapid drying of the material. Such rapid drying is also shown here, however Figs 4 and 5 also demonstrate that decreasing MC significantly increased the pH value, and this corresponded with an increase in temperature from the mesophilic to the thermophilic range (Fig. 2). During the transition from mesophilic to thermophilic temperatures three processes, which result in an increase in pH, are acting simultaneously: (1) decomposition of VFAs during the mesophilic phase, (2) the production of ammonia (NH3), and (3) the dissociation of VFAs. These mechanisms reduce the inhibitory effects of VFAs on micro-organisms and increase their microbial utilisation as an energy source [15].

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Fig. 7 The relationship between temperature and CO2 (A) and temperature and O2 (B)

for all the sampling points at all depths.

Niwagaba et al. [22] observed a similar pattern of response between MC and pH in static laboratory batch composting reactors with increasing addition of FW to mixtures of human faeces containing wood ash. Experiments with a MC >65 % had a minimum pH of ≤6.1 and the temperature did not rise above 43 oC; minimum pH values were generally linearly related to the initial MC of the substrate. However, a marked transitional decline in pH was observed here when the MC increased above 41 % (Fig. 4), which coincided with the upper critical MC value at the intersection between the mean maximum process temperature (54 oC) and the negative linear relationship

A

%CO2 = 15.0 + 0.65 Temp oCR 2 = 0.66P < 0.001

0

5

10

15

20

25

30

35

30 35 40 45 50 55 60 65 70

Compost Temperature (oC)

% C

O2

B

%O2 = 32.1 - 0.56 Temp oC R 2 = 0.69P < 0.01

0

5

10

15

20

25

30 35 40 45 50 55 60 65 70

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% O

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between composting temperature and MC (Fig. 3a). However, initially at least, the process pH was above the minimum value (pH 6.5) associated with the onset of pH inhibition of thermophilic composting. Therefore, a possible alternative explanation for the decrease in process temperature initially observed as compost MC increased is that wetter feedstock materials have higher thermal conductivity and, as the vessel was not perfectly insulated, conductive heat losses may have reduced the rate of heating. For example, other research with rotary biodrying systems [23] showed that thermal losses were responsible for the slower rate of biodrying when the MC of the feedstock was more than approximately 47 %. Also, as increasing MC raises the specific heat of the compost mixture, significantly more energy is required to achieve a given rise in process temperature. Thus, as the MC decreased, the metabolic heat produced by microbial activity increased the temperature of the compost mixture from the mesophilic to the thermophilic range, which, in turn, increased the rate of VFA metabolism and process pH.

A possible explanation for the apparent relationship between pH and MC discussed above is the potential reduction in gas exchange at high moisture contents [24]. Anaerobic micro-sites may develop in compost matrices when gas exchange is restricted, promoting microbial fermentation reactions that also lower the pH value of the mixture. However, there was no evidence of general O2 depletion in the interstitial gas sampled when composting activity was suppressed due to high moisture contents and low pH conditions (Hatch 2 and 3, Fig. 2) as O2 concentrations were sufficient and >8 % during these periods (Hatch 2 and 3, Fig. 6). Therefore, whilst restricted gas exchange may have contributed to the observed low process pH values, it was not considered to be the primary mechanism responsible for the reduction in temperature and acidification response to increasing MC.

As observed elsewhere [11-13, 25], concentrations of CO2 increased and O2 decreased with increasing temperature. Chang et al. [25] showed that very strong linear relationships exist between temperature and CO2 evolution (R2=0.96) and O2 consumption (R2=0.95) when the compost mix is well agitated and aerated. Similar, but weaker relationships were observed here (CO2: R2=0.66; O2: R2=0.69). This may be because aeration and/or agitation of the compost mixture were inadequate when activity was high as indicated by the depletion of O2 in the interstitial gas (Fig. 7).

In the experiments conducted by Beck-Friis et al. [15], CH4 production was observed in situations where O2 concentrations were low, the temperature was in the thermophilic range and the pH was >8.0. The extent of net CH4 production is dependent on the activity of methanogenic and methanotrophic communities during the composting process. Methanogenic activity is slower at mesophilic composting temperatures (37 ºC) compared to thermophilic conditions. However, CH4 oxidation by thermophilic methanotrophs occurs at three times the rate of the CH4 producing capacity of

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methanogenic micro-organisms [26]. This could explain why CH4 concentrations were consistently small irrespective of the composting temperature and O2 depletion.

Decreasing moisture contents between 41 – 48 % were associated with a marked transitional increase in compost pH above 6.0 and a concomitant rise in temperature to thermophilic values. Moisture contents >48 % caused severe acidogenesis and mesophilic temperatures (<45 ºC). The recommended MC for composting is in the range 40 – 60 %. However, under the conditions of this investigation, the upper critical MC for in-vessel composting of FW was at the lower end of this range and equivalent to 40 %.

5. Conclusions

The acidication of FW composting at high MC is frequently linked with process anaerobiosis. However, O2 concentrations in interstitial gas were adequate for aerobic metabolism during periods of process instability and acidification at high moisture contents. Organic acids form during normal aerobic composting metabolism causing an initial drop in pH value. However, the rapid biodegradation of FW means that composting processes treating this feedstock type are particularly susceptible to acid accumulation and pH decline and the results presented here show that process acidification is highly sensitive to increasing MC. However, an alternative explanation to the anaerobic model is that the MC also influences the thermodynamic balance of the composting process and the rate of organic acid metabolism declines with decreasing temperature due to greater heat losses and specific heat of the composting feedstock as MC increases. This creates a feedback mechanism leading to further VFA accumulation and pH reduction, microbial toxicity and process inhibition. The importance of these complex thermodynamic and physiological interactions for FW composting remain unclear and require further study. References 1 WRAP; Waste and Resources Action Programme: Love Food Hate

Waste. http://england.lovefoodhatewaste.com/content/how-much-food-wasted-total-across-uk (2013). Accessed 18 July 2014

2 Lee, P. and Willis, P.: Waste Arisings in the Supply of Food and Drink to Households in the UK. RSC002-005. WRAP, Banbury (2010)

3 Chapagain, A., James, K.: The Water and Carbon Footprint of Household Food and Drink Waste in the UK. Final Report. WRAP, Banbury. http://www.wrap.org.uk/content/water-and-carbon-footprint-household-food-and-drink-waste-uk-1 (2011). Accessed 18 July 2014

4 Environment Agency: Waste Fact Sheet. http://webarchive.nationalarchives.gov.uk/20140328084622/http://www.e

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nvironment-agency.gov.uk/static/documents/Research/Waste_Fact_Sheet_for_SSD_Apr_10.pdf (2010). Accessed 18 July 2014

5 CEU; Council of the European Union: Council Directive of 26 April 1999 on the landfill of waste (99/31/EC). Official Journal of the European Communities No. L182/1-19, Brussels (1999)

6 EPCEU; European Parliament and the Council of the European Union: Regulation (EC) No 1069/2009 of the European Parliament and of the Council of 21 October 2009 laying down health rules as regards animal by-products and derived products not intended for human consumption and repealing Regulation (EC) No 1774/2002 (Animal by-products Regulation). Official Journal of the European Union No L300/1-33 (2009)

7 EC; European Commission: Commission Regulation (EU) No 142/2011 of 25 February 2011 implementing Regulation (EC) No 1069/2009 of the European Parliament and of the Council laying down health rules as regards animal by-products and derived products not intended for human consumption and implementing Council Directive 97/78/EC as regards certain samples and items exempt from veterinary checks at the border under that Directive. Official Journal of the European Union No L54/1-254 (2011)

8 Haug, R.T.: The Practical Handbook of Compost Engineering. Lewis Publishers, Boca Raton (1993)

9 Gilbert, E.J., Riggle, D.S., Holland, F.D.: Large-scale Composting a Practical Manual for the UK. The Composting Association, Northampton (2001)

10 Cebrian Gomez, M.G., Grimes, S.M., Moore, D.: In-vessel composting of food waste - a catering waste management solution. Commun. Waste Resour. Manag. 9, 19-23 (2008)

11 Sundberg, C., Jönsson, H.: Higher pH and faster decomposition in biowaste composting by increased aeration. Waste Manage. 28, 518-526 (2008)

12 Day, M., Krzymien, M., Shaw, K., Zaremba, L., Wilson, W.R., Botden, C., Thomas, B.: An investigation of the chemical and physical changes occurring during commercial composting. Compost Sci. Util. 6, 41-66 (1998)

13 Sundberg, C., Jönsson, H.: Process inhibition due to organic acids in fed-batch composting of food waste - influence of starting culture. Biodegradation 16, 205- 213 (2005)

14 Chang, J.I., Hsu, T-E.: Effects of compositions on food waste composting. Bioresource Technol. 99, 8068-8074 (2008)

15 Beck-Friis, B., Smårs, S., Jönsson, H., Eklind, Y., Kirchmann, H.: Composting of source-separated household organics at different oxygen levels: gaining an understanding of the emission dynamics. Compost Sci. Util. 11, 41-50 (2003)

16 Brinton, W.F.: Volatile organic acids in compost: production and odorant aspects. Compost Sci. Util. 6, 75-82 (1998)

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17 Nishino, T., Nakayama, T., Hemmi, H., Shimoyama, T., Yamashita, S., Akai, M., Kanagawa, T., Hoshi, K.: Acidulocomposting, an accelerated composting process of garbage under thermoacidophilic conditions for prolonged periods. J. Environ. Biotechnol. 3, 33-36 (2003)

18 Sundberg, C., Smårs, S., Jönsson, H.: Low pH as an inhibiting factor in the transition from mesophilic to thermophilic phase in composting. Bioresource Technol. 95, 145-150 (2004)

19 Smårs, S., Gustafsson, L., Beck-Friis, B., Jönsson, H.: Improvement of the composting time for household waste during an initial low pH phase by mesophilic temperature control. Bioresource Technol. 84, 237-241 (2002)

20 Susteco: Neter 30 Composter: Model 30.1 Operation and Maintenance. SustecoAB, Västra Frölunda, Sweden (2004)

21 British Standards Institution: BS EN 13040:2007 Soil improvers and growing media - Sample preparation for chemical and physical tests, determination of dry matter content, moisture content and laboratory compacted bulk density. BSI, London, UK (2008)

22 Niwagaba, C., Nalubega, M., Vinnerås, B., Sundberg, C. and Jönsson, H.: Substrate composition and moisture in composting source-separated human faeces and food waste. Environ. Tech. 30, 487-497 (2009)

23 Skourides, I., Theophilou, C., Loizides, M., Hood, P., Smith, S.R.: Optimisation of an advanced biodrying technology for production of consistent auxiliary fuels from biodegradable municipal waste for industrial purposes. In: Waste 2006: Sustainable Waste and Resource Management, 19-21 September, Stratford-upon-Avon (2006)

24 Tiquia, S.M., Tam, N.F.Y., Hodgkiss, I.J.: Microbial activities during composting of spent pig-manure sawdust litter at different moisture contents. Bioresource Technol. 55, 201-206 (1996)

25 Chang, J.I., Tsai, J.J., Wu, K.H.: Thermophilic composting of food waste. Bioresource Technol. 97, 116-122 (2006)

26 Jäckel, U., Thummes, K., Kämpfer, P.: Thermophilic methane production and oxidation in compost. FEMS Microbiol. Ecol. 52, 175-184 (2005)