bench-scale composting of source-separated human faeces for sanitation

5
Bench-scale composting of source-separated human faeces for sanitation C. Niwagaba a,c, * , M. Nalubega a , B. Vinnerås b,c , C. Sundberg c , H. Jönsson c a Department of Civil Engineering, Makerere University, P.O. Box 7062, Kampala, Uganda b National Veterinary Institute, SE-751 89 Uppsala, Sweden c Department of Energy and Technology, Swedish University of Agricultural Sciences, P.O. Box 7032, SE-750 07 Uppsala, Sweden article info Article history: Accepted 19 June 2008 Available online 8 August 2008 abstract In urine-diverting toilets, urine and faeces are collected separately so that nutrient content can be recy- cled unmixed. Faeces should be sanitised before use in agriculture fields due to the presence of possible enteric pathogens. Composting of human faeces with food waste was evaluated as a possible method for this treatment. Temperatures were monitored in three 78-L wooden compost reactors fed with faeces-to- food waste substrates (F:FW) in wet weight ratios of 1:0, 3:1 and 1:1, which were observed for approx- imately 20 days. To achieve temperatures higher than 15 °C above ambient, insulation was required for the reactors. Use of 25-mm thick styrofoam insulation around the entire exterior of the compost reactors and turning of the compost twice a week resulted in sanitising temperatures (50 °C) to be maintained for 8 days in the F:FW = 1:1 compost and for 4 days in the F:FW = 3:1 compost. In these composts, a reduction of >3log 10 for E. coli and >4log 10 for Enterococcus spp. was achieved. The F:FW = 1:0 compost, which did not maintain 50 °C for a sufficiently long period, was not sanitised, as the counts of E. coli and Enterococcus spp. increased between days 11 and 15. This research provides useful information on the design and operation of family-size compost units for the treatment of source-separated faeces and starchy food residues, most likely available amongst the less affluent rural/urban society in Uganda. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Most nutrients in household waste are contained in the urine and faeces (Jönsson et al., 2005) and when excreta are collected separately using urine-diverting toilets, their nutrient content can be recycled undiluted. Recycling decreases the risk of polluting recipient waters and contributes towards sustainability by closing the nutrient loop. Of the total nutrients in domestic waste, urine contains approximately 80% of the nitrogen (N), about 50% of the phosphorus (P) and nearly 60% of the potassium (K), while the fae- ces contain about 10% of the N, 25% of the P and 20% of the K (Vin- nerås et al., 2006). Urine and faeces contain lower concentrations of heavy metals than farmyard manure and less cadmium than artificial P-fertilizers (Jönsson et al., 2005), making them clean fer- tilisers. Urine contains relatively few pathogens and can be easily disinfected by storage (Schönning et al., 2002). From the hygienic point of view, faeces should be considered to contain pathogens and should therefore be treated before their nutrient content is uti- lized (Schönning and Stenström, 2004; WHO, 2006). The most common faecal treatment to achieve this sanitization is compo- sting (Vinnerås, 2007). Composting is a self-heating microbial aerobic degradation of organic wastes. During the process, carbon dioxide, water and heat are produced and oxygen is consumed. To properly function, com- posting requires well-balanced conditions of moisture and aera- tion. Microbial respiration during composting produces heat (Haug, 1993). The temperature varies within the composting mass, with higher temperatures in the central parts of the mass. To ex- pose the material in the outer low temperature zones to higher temperatures, the material should be mixed (Haug, 1993; Epstein, 1997). To increase temperature, heat loss can be reduced by insu- lation (Haug, 1993; Epstein, 1997). With appropriate insulation, compost temperatures increase. When these higher temperatures (>50°C) are maintained for a sufficient period (1 week), the com- post is sanitized (Schönning and Stenström, 2004; WHO, 2006). In composting, pathogens are killed as a result of elevated tem- peratures and also by competition with the favored thermophilic microbes (Feachem et al., 1983). During thermophilic composting, the temperature effect on pathogen die-off probably dominates. The time-temperature combinations lethal to all pathogens ex- creted in faeces, including the most resistant Ascaris have been re- ported to be: 1 h at 62°C, 1 day at 50°C, and 1 week at 46°C (Feachem et al., 1983). The one possible exception to this is the hepatitis A virus at short retention times (Feachem et al., 1983). When the microbial disinfection capacity of municipal solid waste composting was studied by following the composting process from raw material to mature compost and during long-term storage, no 0956-053X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2008.06.022 * Corresponding author. Address: Department of Civil Engineering, Makerere University, P.O. Box 7062, Kampala, Uganda. Tel.: +256 41 4543152/46 18 671832; fax: +256 41 4543152. E-mail addresses: [email protected], [email protected] (C. Niwagaba). Waste Management 29 (2009) 585–589 Contents lists available at ScienceDirect Waste Management journal homepage: www.elsevier.com/locate/wasman

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Page 1: Bench-scale composting of source-separated human faeces for sanitation

Waste Management 29 (2009) 585–589

Contents lists available at ScienceDirect

Waste Management

journal homepage: www.elsevier .com/locate /wasman

Bench-scale composting of source-separated human faeces for sanitation

C. Niwagaba a,c,*, M. Nalubega a, B. Vinnerås b,c, C. Sundberg c, H. Jönsson c

a Department of Civil Engineering, Makerere University, P.O. Box 7062, Kampala, Ugandab National Veterinary Institute, SE-751 89 Uppsala, Swedenc Department of Energy and Technology, Swedish University of Agricultural Sciences, P.O. Box 7032, SE-750 07 Uppsala, Sweden

a r t i c l e i n f o a b s t r a c t

Article history:Accepted 19 June 2008Available online 8 August 2008

0956-053X/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.wasman.2008.06.022

* Corresponding author. Address: Department ofUniversity, P.O. Box 7062, Kampala, Uganda. Tel.: +25fax: +256 41 4543152.

E-mail addresses: [email protected], ChNiwagaba).

In urine-diverting toilets, urine and faeces are collected separately so that nutrient content can be recy-cled unmixed. Faeces should be sanitised before use in agriculture fields due to the presence of possibleenteric pathogens. Composting of human faeces with food waste was evaluated as a possible method forthis treatment. Temperatures were monitored in three 78-L wooden compost reactors fed with faeces-to-food waste substrates (F:FW) in wet weight ratios of 1:0, 3:1 and 1:1, which were observed for approx-imately 20 days. To achieve temperatures higher than 15 �C above ambient, insulation was required forthe reactors. Use of 25-mm thick styrofoam insulation around the entire exterior of the compost reactorsand turning of the compost twice a week resulted in sanitising temperatures (�50 �C) to be maintainedfor 8 days in the F:FW = 1:1 compost and for 4 days in the F:FW = 3:1 compost. In these composts, areduction of >3log10 for E. coli and >4log10 for Enterococcus spp. was achieved. The F:FW = 1:0 compost,which did not maintain �50 �C for a sufficiently long period, was not sanitised, as the counts of E. coliand Enterococcus spp. increased between days 11 and 15. This research provides useful information onthe design and operation of family-size compost units for the treatment of source-separated faecesand starchy food residues, most likely available amongst the less affluent rural/urban society in Uganda.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Most nutrients in household waste are contained in the urineand faeces (Jönsson et al., 2005) and when excreta are collectedseparately using urine-diverting toilets, their nutrient contentcan be recycled undiluted. Recycling decreases the risk of pollutingrecipient waters and contributes towards sustainability by closingthe nutrient loop. Of the total nutrients in domestic waste, urinecontains approximately 80% of the nitrogen (N), about 50% of thephosphorus (P) and nearly 60% of the potassium (K), while the fae-ces contain about 10% of the N, 25% of the P and 20% of the K (Vin-nerås et al., 2006). Urine and faeces contain lower concentrationsof heavy metals than farmyard manure and less cadmium thanartificial P-fertilizers (Jönsson et al., 2005), making them clean fer-tilisers. Urine contains relatively few pathogens and can be easilydisinfected by storage (Schönning et al., 2002). From the hygienicpoint of view, faeces should be considered to contain pathogensand should therefore be treated before their nutrient content is uti-lized (Schönning and Stenström, 2004; WHO, 2006). The mostcommon faecal treatment to achieve this sanitization is compo-sting (Vinnerås, 2007).

ll rights reserved.

Civil Engineering, Makerere6 41 4543152/46 18 671832;

[email protected] (C.

Composting is a self-heating microbial aerobic degradation oforganic wastes. During the process, carbon dioxide, water and heatare produced and oxygen is consumed. To properly function, com-posting requires well-balanced conditions of moisture and aera-tion. Microbial respiration during composting produces heat(Haug, 1993). The temperature varies within the composting mass,with higher temperatures in the central parts of the mass. To ex-pose the material in the outer low temperature zones to highertemperatures, the material should be mixed (Haug, 1993; Epstein,1997). To increase temperature, heat loss can be reduced by insu-lation (Haug, 1993; Epstein, 1997). With appropriate insulation,compost temperatures increase. When these higher temperatures(>50�C) are maintained for a sufficient period (1 week), the com-post is sanitized (Schönning and Stenström, 2004; WHO, 2006).

In composting, pathogens are killed as a result of elevated tem-peratures and also by competition with the favored thermophilicmicrobes (Feachem et al., 1983). During thermophilic composting,the temperature effect on pathogen die-off probably dominates.The time-temperature combinations lethal to all pathogens ex-creted in faeces, including the most resistant Ascaris have been re-ported to be: 1 h at �62�C, 1 day at �50�C, and 1 week at �46�C(Feachem et al., 1983). The one possible exception to this is thehepatitis A virus at short retention times (Feachem et al., 1983).When the microbial disinfection capacity of municipal solid wastecomposting was studied by following the composting process fromraw material to mature compost and during long-term storage, no

Page 2: Bench-scale composting of source-separated human faeces for sanitation

586 C. Niwagaba et al. / Waste Management 29 (2009) 585–589

re-growth of faecal coliforms or faecal streptococci was encoun-tered (Deportes et al., 1998). Based on this, it was concluded thatfaecal coliforms and faecal streptococci are good candidates fortracking municipal solid waste compost sanitisation (Deporteset al., 1998). Salmonella spp. and Escherichia coli are either inacti-vated or undetectable within 24 h of composting at temperatures>50 �C (Lung et al., 2001; Hess et al., 2004). Composting at 55 �Ckills off E. coli, Listeria and Salmonella spp. within 3 days (Grewalet al., 2006). Using data on survival and inactivation times ofpathogens in compost or manure collected from numerous studies,the Cornell Waste Management Institute (2005) concluded that:(1) Salmonella and E. coli are generally unlikely to survive in com-post where temperatures exceed 50 �C over a period of severaldays to 2 weeks; and (2) faecal coliforms and Enterococcus spp.may be more resistant to high temperature in compost than eitherSalmonella spp. or E. coli.

Studies on composting of source-separated faeces have shownthat a sufficiently high temperature for sanitisation is hard toachieve, as temperatures normally only increase by 10–15 �Cabove the ambient temperature (Karlsson and Larsson, 2000;Björklund, 2002). The faeces in those studies were collected withash and no insulation was provided during the composting. Vin-nerås et al. (2003) composted faeces and food waste in Swedenat a laboratory scale in 1-L and 2-L Dewar vessels, and at pilot-scalein 90-L, 200 mm cell plastic insulated vessels and achieved suffi-ciently high temperatures (>60 �C) to sanitise the material.

The main objectives of the present study were to investigatehow to reach sanitising temperatures when composting faecesand ash mixtures, to determine how the process responded to dif-ferent mixtures of food waste, and to determine how insulation ofsmall family-size compost reactors of approximately 78 L in vol-ume influenced the process. An additional objective was to studythe reduction in faecal indicator organisms (E. coli and Enterococcusspp.) during composting of faeces and food waste.

2. Materials and methods

Composting was performed in three 78-L wooden compostreactors, each with internal dimensions 405 � 405 � 475 mm3

and wall thickness of 25 mm. A 50 � 50 mm2 by 3 mm thick steelangle bar with 5 mm holes at 5 cm centre-to-centre intervals andstaggered along each 50 mm side was placed on the base to pro-vide aeration (Fig. 1). The inside of each reactor was lined with apolyethylene sheet to prevent leakage. A top cover was hingedonto each reactor, so that it could be closed to minimise heat lossfrom the compost. Ambient air from outside the reactor enteredthrough a 20-mm hole at the base, diffused through the holes ofthe angle bar and dispersed into the compost matrix, leavingthrough two 20-mm holes in the top cover. The compost reactors

Equal distances

A

A

Section A-A

Fig. 1. Sketch of the family compost box used in the experiment. Left, Compost boxon wooden stands, Middle: Enlarged plan of the angle bar with staged holes, Right:Section through the enlarged angle.

were placed under shade to protect them from the effects of rain-water. Three experiments (I, II and III) were performed in series,with the knowledge gained in each experiment integrated intothe next one. In Experiment I, no insulation was provided and inExperiments II and III, insulation was provided using 25 mm styro-foam, placed around the reactors sides, at the bottom and at thetop.

The faeces, also containing foreign substances such as paper,sanitary pads and ash, were composted. To increase the organicfraction, the material was sieved prior to composting on a 6.35-mm sieve. Different proportions of sieved faeces were mixed withfood waste to give compost substrates containing faeces: foodwaste (F:FW) in wet weight mix ratios of F:FW = 1:0, 3:1 and1:1. These substrate mixtures were used in Experiments I, II andIII, which were set up and operated according to Table 1.

To prevent excessive drying of the compost during the experi-ments, the moisture lost was replaced by sprinkling water fol-lowed by mixing using a compost agitator tool for approximately10 min. Water was sprinkled after visually inspecting the compostmaterial to check its dryness. This was performed once a week inExperiments I and II, and twice a week in Experiment III for thefirst 2 weeks and thereafter once a week. Table 2 shows the differ-ent substrates used in Experiments I, II and III by wet weight andtype.

Temperatures were measured daily using a portable digitalthermometer (Clas Ohlson Model 307, Taiwan). On each measure-ment occasion, temperature was recorded at five different sam-pling sites, in each of the four corners and in the middle of thecompost. At each point, the temperature probe was inserted intothe compost and moved up and down to 10–50 mm from thetop. The highest temperature reading of the thermometer was re-corded for each point, often within 20 mm from the top. In Exper-iments I and II, temperatures were measured once a day (at09.00 h). In Experiment III, temperature changes were more rapidand readings were taken three or four times a day (mainly at09.00, 12.00, 15.00 h and often also at 18.00 h). Ambient air tem-perature was not measured. However, in the area where the exper-iment was conducted (Kampala, Uganda), daily temperatures werein the range 27–30 �C during the study period.

Grab samples from different parts and depths of the compostreactor were handpicked at four locations of the compost andmixed by hand to get a representative sample of about 200 g.The sample was divided and distributed for analysis of pH, E. coliand Enterococcus spp. For pH, 5 g of the compost sample was mixedwith 25 ml of de-ionised water. The pH was measured using a pHmeter (HANNA, HI 98128, USA). E. coli and Enterococcus spp. wereanalysed using Chromocult and Esculin Azide agar, respectively.From a tenfold dilution series, 0.1 ml aliquot were spread ontothe agar surface and incubated for 24 h at 44 �C followed by enu-meration (FAO, 2001).

3. Results

Figs. 2a–c show temperature changes over time in ExperimentsI, II and III, respectively. The maximum temperature increase (allthree reactors taken into consideration) above that of the sur-rounding air was approximately 14 �C in Experiment I (non-insu-lated), about 25 �C in Experiment II and over 35 �C in ExperimentIII.

In Experiment III, where the reactors were insulated, tempera-tures in all three reactors increased rapidly. In the F:FW = 3:1 com-post, the maximum temperature was 67 �C on day 2 (Fig. 2c).However, this temperature was not maintained. Beginning fromday 5, the F:FW = 1:1 compost was the hottest, with temperaturesreaching a maximum of 62 �C on day 9. The F:FW = 3:1 compostmaintained >50 �C for 4 days from the start of the composting.

Page 3: Bench-scale composting of source-separated human faeces for sanitation

Table 1Description of experiments and substrates

Parameter Experiment I (December 04) Experiment II (Jan–Feb 05) Experiment III (March–May 05)

Insulation No insulation 25 mm thick styrofoam 25 mm thick styrofoamFood waste mix Maize meal (Posho) and beans Maize meal (Posho), beans and fruit peelings of orange,

avocado, lemon, jackfruit, tangerine, watermelon andpassion fruits

Peas, rice, bananas, beans, banana leaves, maize meal(posho), sweet potatoes, cassava, potatoes

Inoculation 500 mL of yoghurt per reactor and3 kg of sawdust per reactor

1 kg of old compost and 2.25 kg of sawdust per reactor 1 kg of old compost and 2.25 kg of sawdust perreactor

Initial preparation Material was mixed in thereactors using a spade whilesprinkling water

Material was mixed in the reactors using a spade whilesprinkling water

Material was first mixed on a polythene sheet usinga spade while sprinkling water and then placed inreactors

Mixing Once a week Once a week Twice a week for the first two weeks then once aweek

Table 2Amounts of faeces (F), food waste (FW) and sawdust used in the three compost experiments

Ratio Experiment I Experiment II Experiment III

F (kg) FW (kg) Sawdust (kg) F (kg) FW (kg) Sawdust + old compost (kg) F (kg) FW (kg) Sawdust + old compost (kg)

1:0 60 0 3 45 0 2.25 + 1 45 0 2.25 + 13:1 45 15 3 33.75 11.25 2.25 + 1 33.75 11.25 2.25 + 11:1 30 30 3 22.5 22.5 2.25 + 1 22.5 22.5 2.25 + 1

C. Niwagaba et al. / Waste Management 29 (2009) 585–589 587

The F:FW = 1:1 compost maintained >50 �C for 8 days in a row(from day 5 to day 13).

In all experiments, the measured pH values were between 6.9and 9.4. In general, the pH increased with an increasing proportionof faeces in the mix and with increasing stability of the compost.

In all compost reactors of Experiment I, E. coli was detected(detection limit 102 cfu/g), in all samples in concentrations of103–105 cfu/g, irrespective of when the sample was taken. InExperiment II, E. coli was detected in all samples in concentrationsof 103 cfu/g with no tendency for a reduction. In Experiment III,E. coli was below the detection limit in all of the mixtures on day7 (Fig. 3a), but in the F:FW = 1:0 compost mixture, 4log10 cfu/gwere detected on day 15.

The Enterococcus spp. counts in Experiment I were almost thesame in all the samples taken at the end of the experiment as inthe starting samples. After 19 days of treatment, Enterococcusspp. remained detectable (detection limit 102 cfu/g) in all compostsin Experiment II. In Experiment III, there was a decrease in Entero-coccus spp. with time until there was no detection in F:FW = 3:1and F:FW = 1:1 composts. In F:FW = 1:0 compost, Enterococcusspp. was detectable throughout the study and it increased from3log10 to 5.5log10 between day 11 and day 15 (Fig. 3b).

4. Discussion

Experiment I was characterised by a small temperature rise inall of the composts (Fig. 2a). In this experiment, the reactors werenon-insulated. The results of the insufficient temperature rise inExperiment I agree with those of Björklund (2002), who obtaineda maximum temperature of 34 �C (only about 15 �C higher thanambient temperature) when composting faeces (together withsoil/lime mixture, mixed with garden soil, food waste and gardenwaste) in non-insulated 50-L reactors in Mexico. The results alsoagree with those of Karlsson and Larsson (2000), who achievedonly 10 �C above ambient temperature when composting faeces/ash mixture in four non-insulated compost piles in Ethiopia.

The increased elevation in temperature in Experiment II wasattributed to the added insulation (Table 1). In the F:FW = 1:0 sub-strate, the temperature began to decrease after day 2 (Fig. 2b), sug-gesting that the most easily degradable organic matter could havebeen quickly used up. However, the mixtures containing larger

proportions of food waste did not reach higher temperatures orsustain elevated temperature for long, indicating that the easilyavailable organics were probably low in the fruit peelings presentin this waste.

In Experiment III, the F:FW = 3:1 and 1:1 composts reached hightemperatures. A similar temperature-time relationship was ob-tained by Vinnerås et al. (2003) when composting a mixture of fae-ces and food waste in Sweden. Composting was performed in a 90-L bin, provided with 200 mm cell plastic insulation on all sides.Ambient air temperatures were 10 �C (±3 �C). In the experimentby Vinnerås et al. (2003), the material composition on a dry matterbasis was 21% faeces, 60% food waste and 19% amendment (oldcompost).

Co-composting of faeces and food waste in well-insulated reac-tors seems to be able to produce sufficiently high temperatures(Fig. 2c) for a sufficiently long period to achieve safe sanitation(Figs. 3a and b). However, the composition of the food waste alsoseems to play an important role in the temperature change. InExperiment II, fruit peelings constituted the largest fraction ofthe food waste and hence the food waste mixture in this experi-ment may have had less readily available carbon compared withthe kitchen food wastes used in Experiment III. Furthermore, thefood waste composition used in Experiment II is not common espe-cially amongst poor communities, who may not have significantamounts of fruit in their diet.

To maintain a high temperature for the time needed for sanita-tion, food waste seems to be required as the easily available organ-ic content in the faecal matter seems to be too low for maintainingthe high temperature for a sufficient period. This was shown inExperiment III, where the F:FW = 1:0 compost reached the highesttemperature after just 2 days but then declined to temperaturesbelow 50 �C during the rest of the study, while the composts con-taining food waste maintained high temperatures for longer(Fig. 2c). In experiments using larger volume (216-L) reactors thatwere also insulated with 75 mm styrofoam, it was possible to reachhigh temperatures that were maintained for up to 1 week withsubstrates containing faeces and ash only (Niwagaba et al.,2008). However, in that study too, addition of food waste increasedboth the temperature and the duration of high temperatures andthe need to have insulated reactors in order to reach sufficientlyhigh temperatures for full sanitation was highlighted.

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Fig. 2. Temperature changes in (a) Experiment I (before insulation) and in (b)Experiment II and (c) Experiment III (after insulation) [Showing mean, maximumand minimum of five measurements taken on each occasion].

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Fig. 3. Changes in (a) E. coli and (b) Enterococcus spp. (log10 c.f.u/g) over time inExperiment III.

588 C. Niwagaba et al. / Waste Management 29 (2009) 585–589

When faeces with ash (F:FW = 1:0) were composted, the initialtemperature rise was much faster than when substrates with high-er proportions of food waste (F:FW = 1:1) were composted. Gener-ally the pH in food waste is low (Eklind et al., 1997). This isattributed to the presence of short-chain organic acids, mainly lac-tic and acetic acids (Beck-Friis et al., 2001). The concentrations ofthese acids could decrease during the initial stages of composting.This often inhibits the composting process (Sundberg et al., 2004).In our experiments, the ash in the faeces probably buffered the pHto a neutral or basic level, giving the process a fast start as no acidlag phase appears to have occurred for the F:FW = 1:0 compostsdue to the rapid temperature increase. The temperature increasesin the composts with food waste substrates (F:FW = 3:1 and 1:1)were slower. The pH in the composts (measured every 3–5 days)was in the range 6.9–9.4 throughout the study. The lowest pH val-ues were measured for the F:FW = 1:1 compost in Experiment I on

the initial day. Studies on inclusion of more ash with a high pHwith the faeces showed that the composting process was ham-pered when the pH was greater than 10 (Vinnerås et al., in press).Therefore, having some ash in the substrate may improve thespeed of the composting process but if the system becomes too ba-sic, the composting is affected.

In these experiments, the proportion of ash was reduced bysieving using a 6.35-mm aperture sieve. This practice is unhygienicand risky. Possible alternative ways of increasing the organic frac-tion of the substrate, which need to be investigated further, are toreplace some of the ash used as additive material during the collec-tion phase with a mixture of ash and sawdust or adding appropri-ate amounts of food waste to the faeces mixture to increase theorganic fraction and bring the pH of the starting substrate to a suit-able value (pH between 6.5 and 9) (Sundberg et al., 2004).

As a rule of thumb for thermal sanitation of faecal matter, treat-ment above 50 �C for 1 week is recommended (Schönning andStenström, 2004). Failure to reach sanitising temperatures in anyof the compost reactors of Experiment I and to maintain these tem-peratures for at least 1 week in any of the compost reactors ofExperiment II suggests that safe sanitation was not achieved. Thiswas confirmed by the analyses of indicator bacteria in the twoexperiments. In Experiment I, there seems to have been no reduc-tion of either E. coli or Enterococcus spp. Only some reduction ofE. coli was detected in Experiment II. The results of Experiment Iconfirm 50 �C as the threshold to attain sanitation, while Experi-ment II reinforces the requirement that it is not enough to justreach 50 �C but rather to maintain it for a sufficient period to attainsanitation.

The F:FW = 1:1 compost mix in Experiment III produced tem-peratures >50 �C for 8 days and in this compost no E. coli was de-tected in any of the last three enumerations or no Enterococcusspp. in the last two enumerations. In the F:FW = 3:1 compost,>50 �C was maintained for only 4 days and no E. coli or Enterococcusspp. were detected in any of the three enumerations taken afterday 7. The margin above 50 �C was high (i.e., maximum 67 �Cand >60 �C for at least 1 day), suggesting that the temperature-time is important and that sanitation can be achieved during short-

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C. Niwagaba et al. / Waste Management 29 (2009) 585–589 589

er periods at high temperatures, as reported previously by Fea-chem et al. (1983). These results also agree with other studiesreporting that composting at 50 �C to 55 �C results in die-off ofE. coli and Enterococcus spp. in 3 days to 2 weeks (Jepsen et al.,1997; Cornell Waste Management Institute, 2005; Grewal et al.,2006). In the F:FW = 1:0 compost, E. coli was detected in concentra-tions of 104 cfu/g on day 15, suggesting either re-growth or non-homogeneous survival in the compost, as some parts within themixture attained lower temperatures. These results are in line withfindings by Vinnerås (2007), showing that despite reaching opti-mum sanitization temperatures (>50 �C), there is no guarantee thatviable pathogenic organisms have been eradicated. This potentialsurvival is largely attributed to the heterogeneous nature of thecompost pile or bin and stresses the importance of mixing duringcomposting.

5. Conclusions

It is important to provide insulation for compost reactors, evenunder tropical conditions where ambient air temperatures may behigh (>25 �C), since sanitising temperatures (>50 �C) were not at-tained in any experiment that lacked insulation. In this study, 25mm styrofoam insulation placed around the outside of the 78-Lcompost reactors helped to achieve and maintain sanitisingtemperatures.

Composting of substrates consisting of faeces and kitchen foodwaste in equal wet weight ratios can achieve sufficient tempera-tures for sufficient duration to attain sanitation. In ExperimentIII, where sanitising temperatures (�50 �C) were maintained for 4days in the F:FW = 3:1 compost and for 8 days in the F:FW = 1:1compost, E. coli was not detectable from day 7 and Enterococcusspp. was not detectable from day 11. E. coli and Enterococcus spp.were detected at the end of the measurement period in samples ta-ken from the F:FW = 1:0 compost, which did not hold �50 �C for asufficient period (this temperature having been measured on onlya few occasions between days 2 and 3), implying that this compostwas not sanitised.

The detection of E. coli on day 15 in the F:FW = 1:0 compost, fol-lowing no detection in a sample taken on day 11, seems to implythat even if no indicator organisms are detected on some samplingoccasions, this is no guarantee of no organisms in the treated prod-uct, as non-homogeneous material and temperature distributioncombined with the risk of re-growth can result in high bacterialcounts at a later stage.

Acknowledgements

The authors wish to thank staff at the Microbiology Laboratory,Faculty of Veterinary Medicine, and the Soil Science Laboratory,Faculty of Agriculture, at Makerere University who assisted withsample analyses. We also acknowledge and thank Beatrice Mag-umba and Dauda Kiwanuka for their participation in the experi-mental work. The study was financed by Sida/SAREC and theDirectorate of Water Development (DWD).

References

Beck-Friis, B., Smårs, S., Jönsson, H., Kichmann, H., 2001. Gaseous emissions ofcarbon dioxide, ammonia and nitrous oxide from organic household waste in acompost reactor under different temperature regimes. J. Agric. Eng. Res. 78 (4),423–430.

Björklund, A., 2002. The potential of using thermal composting for disinfection ofseparately collected faeces in Cuernacava, Mexico. Minor Field Studies No 200.Swedish University of Agricultural Sciences, International Office. ISSN 1402-3237.

Cornell Waste Management Institute, 2005. Pathogen analysis of NYSDOT road-killed deer carcass in compost facilities. h<ttp://cwmi.css.cornell.edu/tirc/Appendix10.htm> (accessed 08.04.06).

Deportes, I., Benoit-Guyod, J.L., Zmirou, D., Bouvier, M.C., 1998. Microbialdisinfection capacity of municipal solid waste (MSW) composting. J. Appl.Microbiol. 85, 238–246.

Eklind, Y., Beck-Friis, B., Bengtsson, S., Ejlertsson, J., Kirchmann, H., Mathisen, B.,Nordkvist, E., Sonesson, U., Svensson, B.H., Torstensson, L., 1997. Chemicalcharacterisation of source-separated organic household waste. Swedish J. Agric.Res. 27, 167–178.

Epstein, E., 1997. The Science of Composting. CRC Press LLC, Boca Raton, London,New York, Washington DC, ISBN 1-56676-478-5.

FAO, 2001. Manual of Food Quality Control: 4 Microbiology Analysis, 10th ed.Bucham Publishers, West Sussex, UK.

Feachem, R.G., Bradley, D.J., Garelick, H., Mara, D.D., 1983. Sanitation and Disease.Health Aspects of Excreta and Wastewater Management. World Bank Studies inWater Supply and Sanitation. John Willey and Sons, New York.

Grewal, S.K., Rajeev, S., Sreevatsan, S., Michel, F.C., 2006. Persistence ofMycobacterium avium subsp paratuberculosis and other zoonotic pathogensduring simulated composting, manure packing and liquid storage of dairymanure. Appl. Environ. Microbiol. 72 (1), 565–574.

Haug, R.T., 1993. The Practical Handbook of Compost Engineering. Lewis Publishers,Boca Raton, Washington DC. ISBN 0-87371-373-7.

Hess, T.F., Grdzelishvili I, Sheng H., Hovde, C.J., 2004. Heat inactivation of E. coliduring manure composting. Compost Sci. Util. 12 (4), 314–322.

Jepsen, S.-E., Krause, M., Grüttner, H., 1997. Reduction of faecal Streptococcus andSalmonella by selected treatment methods for sludge and organic waste. WaterSci. Technol. 36 (11), 203–210.

Jönsson, H., Baky, A ., Jeppsson, U., Hellström, D., Kärrman, E., 2005. Composition ofUrine, Faeces, Greywater and Bio-waste – for Utilisation in the URWARE Model.Report 2005:6, Urban Water, Chalmers. Sweden. <www.urbanwater.org>.

Karlsson, J., Larsson, M., 2000. Composting Latrine Products in Addis Ababa,Ethiopia. Minor Field Studies No. 32. Luleå: Luleå University of Technology.

Lung, A.J., Lin, C.M., Kim, J.M., Marshall, M.R., Nordstedt, R., Thompson, N.P., Wei, C.I.,2001. Destruction of Escherichia coli O157:H7 and Salmonella Enteritidis in cowmanure composting. J. Food Protect. 64 (9), 1309–1314.

Niwagaba, C., Nalubega, M., Vinnerås, B., Sundberg, C., Jönsson, H. 2008. Compostingof source separated human faeces. Manuscript.

Schönning, C., Stenström, T.A., 2004. Guidelines for the Safe Use of Urine and Faecesin Ecological Sanitation. Report 2004-1. Ecosanres, SEI, Sweden.<www.ecosanres.org>.

Schönning, C., Leeming, R., Stenström, T.A., 2002. Faecal contamination of source-separated human urine based on the content of faecal sterols. Water Res. 36,1965–1972.

Sundberg, C., Smårs, S., Jönsson, H., 2004. Low pH as an inhibiting factor in thetransition from mesophilic to thermophilic phase in composting. Biores.Technol. 95, 145–150.

Vinnerås, B., 2007. Comparison of composting, storage and urea treatment forsanitising of faecal matter and manure. Biores. Technol. 98, 3317–3321.

Vinnerås, B., Björklund, A., Jönsson, H., 2003. Thermal composting of faecal matteras treatment and disinfection method – Laboratory-scale and pilot-scalestudies. Biores. Technol. 88 (1), 47–54.

Vinnerås, B., Palmquist, H., Balmér, P., Weglin, J., Jensen, A., Andersson, Å., Jönsson,H., 2006. The characteristics of household wastewater and biodegradable waste– a proposal for new Swedish norms. Urban Water 3 (1), 3–11.

Vinnerås, B., Jönsson, H., Niwagaba, C., Nordin, A., Nalubega, M., in press. Systemsfor urine collection and for treatment of faecal matter for nutrient recyclingfrom urban areas. EcoSanRes. SEI, Stockholm, Sweden.

WHO, 2006. Guidelines for the Safe Use of Wastewater, Excreta and Greywater.Excreta and Greywater Use in Agriculture. Vol. 4, ISBN 92 4 154685 9.