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Substrate composition and moisture in compostingsource‐separated human faeces and food wasteC. Niwagaba a b , M. Nalubega a , B. Vinnerås b c , C. Sundberg b & H. Jönsson b da Department of Civil Engineering , Makerere University , P.O. Box 7062, Kampala, Ugandab Department of Energy and Technology , Swedish University of Agricultural Sciences , P.O.Box 7032, SE‐750 07, Uppsala, Swedenc National Veterinary Institute , SE‐751 89, Uppsala, Swedend Stockholm Environment Institute , Kräftriket 2B, 10691, Stockholm, SwedenPublished online: 06 Apr 2009.

To cite this article: C. Niwagaba , M. Nalubega , B. Vinnerås , C. Sundberg & H. Jönsson (2009) Substrate composition andmoisture in composting source‐separated human faeces and food waste, Environmental Technology, 30:5, 487-497, DOI:10.1080/09593330902788236

To link to this article: http://dx.doi.org/10.1080/09593330902788236

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Environmental Technology

Vol. 30, No. 5, 14 April 2009, 487–497

ISSN 0959-3330 print/ISSN 1479-487X online© 2009 Taylor & FrancisDOI: 10.1080/09593330902788236http://www.informaworld.com

Substrate composition and moisture in composting source-separated human faeces and food waste

C. Niwagaba

a,b

*, M. Nalubega

a

, B. Vinnerås

b,c

, C. Sundberg

b

and H. Jönsson

b,d

a

Department of Civil Engineering, Makerere University, P.O. Box 7062, Kampala, Uganda;

b

Department of Energy and Technology, Swedish University of Agricultural Sciences, P.O. Box 7032, SE-750 07, Uppsala, Sweden;

c

National Veterinary Institute, SE-751 89 Uppsala, Sweden;

d

Stockholm Environment Institute, Kräftriket 2B, 10691, Stockholm, Sweden

Taylor and Francis

(

Received 5 November 2008; Accepted 30 January 2009

)

10.1080/09593330902788236

The composting of a faeces/ash mixture and food waste in relative proportions of 1:0, 1:1 and 1:3 was studied in threesuccessive experiments conducted in Kampala, Uganda in 216 L reactors insulated with 75 mm styrofoam or notinsulated. The faeces/ash mixture alone exceeded 50

°

C for

≤

12 days in insulated reactors, but did not reach ormaintain 50

°

C in non-insulated reactors. Inclusion of food waste kept temperatures above 50

°

C for over two weeksin insulated reactors except when the substrate was too wet.

Escherichia coli

and total coliform concentrationsdecreased below detection in material that exceeded 50

°

C for at least six days.

Enterococcus

spp. decreased belowdetection in material that exceeded 50

°

C for at least two weeks, but remained detectable after 1.5 months in materialthat exceeded 50

°

C for less than two weeks, suggesting that a period of at least two weeks above 50

°

C, combinedwith mixing, is needed to achieve sanitation. Initially substrates that were too wet proved a challenge to compostingand ways of decreasing substrate moisture should be investigated. The results obtained are applicable to themanagement of small- to medium-scale composting of faeces/ash and food waste at household and institution levels,e.g. schools and restaurants.

Keywords:

composting; faeces; food waste; insulation; temperature

Introduction

In urine-diverting toilets, urine and faeces are collectedseparately in order to simplify the safe reuse of nutrientsin the excreta. The majority of the nutrients present inhousehold waste originate from urine, while faecesprovide (by wet weight in household waste, includinggrey water) about 10% of the nitrogen, 25% of thephosphorus and 20% of the potassium [1]. From theperspective of health risk, faeces should always beconsidered to contain pathogens [2,3]. To break thefaecal–oral route of disease transmission by any patho-genic microorganisms that may be present, faecesshould be sanitized [3,4], and one method of achievingthis is by thermophilic composting [5].

During composting, oxygen is consumed and carbondioxide, water and heat are produced as a result ofmicrobial respiration [6,7]. To achieve and maintainhigh temperatures, the heat generated should remainwithin the system for as long as possible. In large-scalecomposting, the outer part of the heap acts as an insula-tor, decreasing heat loss, but small compost heaps needto be insulated to retain enough of the heat to raise theirtemperature to sanitising levels [5,8,9]. Thorough

mixing of the material on several occasions duringcomposting is important to ensure that all compost partsare exposed to high temperatures [7]. Sufficiently hightemperatures for sufficiently long periods result in aproduct that is free from pathogens [2,3]. In addition,compost contains plant nutrients, especially nitrogen,phosphorus and potassium, and is a good soil condi-tioner, adding humus and organic matter to improveagricultural productivity [7,10,11,12].

The microbial activity during composting is affectedby pH, moisture content, oxygen content, temperatureand substrate composition [7,10,13]. Well-functioningthermophilic composting occurs when the pH isbetween 6.5 and 9.4 [9,14,15,16,17]; moisture contentis between 40% and 60% [7,10,17]; and optimumtemperature (at which maximum decomposition occurs)is in the range 50–67

°

C [13,18,19]. The degradationrate during composting varies according to the organicmatter constituents [20].

Thermal sanitising temperatures (>50

°

C) wereachieved when composting source-separated faecesmixed with food waste in 1 L and 2 L Dewar vessels,and in 90 L pilot-scale vessels insulated with 200 mm

*Corresponding author. Email: charles.niwagaba@et.slu.se

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cell plastic [5]. That study included food waste in orderto increase the content of easily degradable organicsand to improve substrate structure. The insulation wasused to decrease the heat loss and hence increasetemperatures and make the temperature distributionmore uniform throughout the material. Severalresearchers have found that without insulation, thetemperature of faecal composts is less than 10–15

°

Cabove the ambient temperature [5,8,9]. Successful sani-tising composting of faeces/ash mixtures in 78 L reac-tors insulated with 25 mm styrofoam required theinclusion of food waste on a 1:1 wet weight basis [9].This mixture maintained sanitising temperatures formore than one week, and neither

E. coli

nor

Enterococ-cus

spp. were detected from day 7 and 11, respectively.Faeces:food waste ratios of 1:0 and 3:1 on a wet weightbasis did not maintain sanitising temperatures suffi-ciently long (more than one week) to attain safe sanita-tion according to [2].

The objective of this study was to evaluate the possi-bility of composting faecal matter collected with ash,with and without addition of food waste, in a 216 Lreactor and reaching sanitising temperatures (above

50

°

C) for a sufficiently long period for production of ahygienically safe end-product.

Materials and methods

Three experiments were conducted in succession, inthree plywood reactors in Experiment 1 and in fivereactors in Experiments 2 and 3.

Compost reactors

Plywood reactors of internal dimensions 600

×

600

×

600 mm

3

(216 L) with 9 mm thick walls were used. Thebase of the reactors consisted of 50

×

50 mm

2

by 3 mmthick mild steel cross-plates with 5 mm holes at 5 cmcentre-to-centre spacing to provide for aeration(Figure 1). Ambient air entered through the hole at thebase of the reactor, diffused through the holes on thecross-plates and dispersed into the compost matrix,emerging from the top.

Figure 1. Diagram of the plywood reactor showing the top cover, the mild steel cross-plates and air ventilation holes.

All reactors in Experiment 1 and four of the five reac-tors in Experiments 2 and 3 were insulated with 75 mmof styrofoam on all sides, including the top cover. The

600mm

600mm

75mm+9mm

75mm+9mm

600m

m

Top insulated cover

Insulated plywood reactor

Air in

600mm

600mm

75mm+9mm

75mm+9mm

600m

m

Top insulated cover

Insulated plywood reactor

Air in

Figure 1. Diagram of the plywood reactor showing the top cover, the mild steel cross-plates and air ventilation holes.

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top cover had a loose fit to the side walls, allowing venti-lation air to escape. The reactors were placed under a roofcover to protect them from direct sunlight and rain.

Sample collection and preparation

Source-separated faeces containing toilet paper, sanitarypads and wood ash (applied to cover faeces during toiletuse) were collected from urine-diverting toilets inMasaka and Kampala, Uganda. To increase the propor-tion of organic matter, loose ash was removed by sievingthrough a quarter-inch (6.35 mm) aperture sieve. Foodwaste was collected from three schools located inKampala. The expected waste production, by wetweight, in Kampala is 250–350 g p

−

1

d

−

1

[4] of faeces and0.9–1.1 kg p

−

1

d

−

1

of solid waste, of which approximately80% is organic [21]. The design of urine-diverting drytoilets in Uganda assumes application of ash or otheradditive in approximately a 1:1 wet weight (w/w) ratioto faeces [22]. The w/w ratio of collectable source-sepa-rated faeces/additive to food waste would thus beapproximately 1:1–3. The characteristics of thesubstrates used in Experiments 1–3 are shown in Table 1.

A mix of food waste that was 1–14 days old wasused in the experiments, as it took 14 days in total toaccumulate the amounts needed, with approximatelyequal quantities collected each day. The moisturecontent of the starting substrates was estimated by thefist test, in which a fistful of material was squeezedinto a ball that held its form but crumbled under slightpressure. The moisture content of overly dry substrates

was corrected by sprinkling with water, followed bymixing and repetition of the fist test.

Measured quantities of sieved faeces/ash and foodwaste were spread on a polyethylene sheet and mixedusing spades to a homogeneous mixture containingfaeces/ash:food-waste in wet mix ratios of 1:0, 1:1 and1:3 according to volume (V) in Experiment 1, andaccording to weight (W) in Experiments 2 and 3.

To provide structure and inoculum for process start-up, 2 kg of sawdust and 1 kg of old compost were addedto each mixture in Experiments 1 and 2, and 5 kg ofsawdust and 2 kg of old compost, to each mixture inExperiment 3.

Process monitoring and temperature measurement

Water was sprinkled on the surface, followed by mixingwith a spade when needed according to visual observa-tion. Water was added once or twice a week in all runsin Experiment 1, in runs W1:0a, W1:1a and W1:0b, andoccasionally also in W1:0aN and W1:0bN (see Table 1for an explanation of the different runs). No water wasadded in W1:3a, W1:3aF, W1:1b, W1:3b and W1:3bF,as these runs never heated up and their moisture contentremained high.

Compost temperatures were measured two to fourtimes a day (at 9.00 and 12.00, and occasionally also at15.00 and 18.00) using a portable digital thermometer(Model 307, Taichung, Taiwan). The temperature wasmeasured at five points within each reactor: in the fourcorners (1 cm from each of the reactor walls) and in the

Table 1. Characteristics of the substrates used in compost reactor runs in Experiments 1, 2 and 3.

Exp Run

Faeces/ash mixture: Food

waste ratio

Wet weight

(kg)

Bulk density

(kg m

−

3

)MC

(% ww) C/NVS

(% TS) pHOrganic C (%TS)

1 V1:0 1:0 165 782 45 – 57 9.8 –V1:1 1:1 169 801 52 – 67 7.9 –V1:3 1:3 190 900 57 – 69 7.3 –

2 W1:0aN

a

1:0 173 801 47 9 23 9.5 8W1:0a 1:0 171 789 48 7 23 9.3 12W1:1a 1:1 161 813 63 10 39 7.7 15W1:3a 1:3 185 934 70 15 46 6.4 23

W1:3aF

b

1:3 203 1025 72 21 46 6.0 21

3 W1:0bN

a

1:0 174 803 43 11 32 9.7 20W1:0b 1:0 172 796 43 11 32 9.7 20W1:1b 1:1 167 843 67 13 41 8.6 17W1:3b 1:3 199 1005 69 17 49 8.5 28

W1:3bF

b

1:3 203 1051 73 14 46 7.6 24

MC = moisture content, VS = volatile solids, C = carbon, N = nitrogen, C/N = carbon to nitrogen ratio, ww = wet weight, TS = total solids.

a

Runs W1:0aN and W1:0bN were not insulated, while the rest of the runs were insulated.

b

The food waste in runs W1:3aF and W1:3bF was fresh (collected and used on the same day), while food waste was 1–14 days old in all other runs where food waste was included.

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middle. At each location, the thermometer probe wasinserted into the compost and moved up and down. Thehighest temperature reading of the thermometer wasrecorded, often within 50–150 mm from the top of thecompost. Ambient air temperature was also recorded.

Sampling and analysis

Grab samples from three locations, near the top of thereactor, in the middle and close to the bottom, weremixed by hand to get a representative sample of about200 g from each reactor. For pH measurements, 5 g ofmixed material was dissolved in 25 mL deionised waterand shaken, after which it was left to stand for about 25minutes. The pH was measured using a WTW inoLab730 pH meter (Weilheim, Germany). The moisturecontent of approximately 30 g sub-samples was deter-mined by oven-drying at 105

°

C for 24 h. The sampleswere then incinerated in a muffle oven at 550

°

C (start-ing at room temperature) for >6 h for determination ofvolatile solids.

Escherichi coli

and total coliforms wereanalysed using Chromocult Agar, and

Enterococcus

spp., using Esculin Azide Agar. From a ten-fold dilutionseries, 0.1 mL aliquots were spread on to the agarsurface and incubated for 24 h at 37

°

C for totalcoliforms and at 44

°

C for the other two microorgan-isms, followed by enumeration [23].

The proportion of degraded material was estimatedfrom the decreasing proportion of volatile solids in thefinal product (Equation (1)) according to [7]:

where

k

m

is the percentage of volatile solids degradedduring the treatment,

VS

m

% is the volatile solids contentof the mixture substrates as the percentage of totalsolids (TS), and

VS

p

% is the product volatile solidscontent as the percentage of TS.

Results

The main material and process/product characteristicsof the substrates used in Experiments 1–3 are presentedin Table 2.

Temperature and insulation

The temperature data recorded are summarised inTable 3, where temperature maxima in the various runsare compared with the temperature at the centre of thecompost reactor and its four corners and with the ambi-ent temperature.

Temperatures increased quickly in all reactors inExperiment 1 (Figure 2). The experimental run with thehighest proportion of food waste (V1:3) was initiallythe hottest. The experimental run without food waste(V1:0) was the coolest throughout the study.

Figure 2. Temperatures in reactors V1:0, V1:1 and V1:3 in Experiment 1 [mean, maximum and minimum of five measurements taken on each occasion]. Amb. is ambient temperature.

In Experiment 2, the temperatures in runs W1:0aand W1:1a varied (Figure 3). Runs W1:0aN, W1:3a,and W1:3aF did not reach sanitising temperatures andtheir temperature stayed on average 2–5

°

C above ambi-ent temperature over the measurement period (data notshown).

Figure 3. Temperatures in reactors reaching sanitising temperatures in Experiment 2 [mean, maximum and minimum of five measurements taken on each occasion]. Amb. is ambient temperature.

In Experiment 3, run W1:0b self-heated as shown inFigure 4. Runs W1:0bN, W1:1b, W1:3b and W1:3bFdid not attain sanitising temperatures (>50

°

C), and wereoccasionally colder than ambient air (data not shown).

Figure 4. Temperatures in a reactor reaching sanitising temperatures in Experiment 3 [mean, maximum and minimum of five measurements taken on each occasion]. Amb. is ambient temperature.

Table 2. Process and product characteristics of materials used in compost reactor runs in Experiments 1, 2 and 3.

Run pH

min

pH

mean

pH

max

pH

final

VS

final

, (%TS)

MC

mean

(%ww)

Degraded VS (% of initial VS)

V1:0 9.1 9.6

±

0.2 10.0 9.7 48 37

±

4 30V1:1 7.9 9.3

±

0.5 9.9 9.7 44 41

±

8 61V1:3 7.3 9.5

±

0.6 10.0 9.9 38 37

±

8 72W1:0aN 9.3 9.5

±

0.1 9.7 9.7 18 43

±

4 27W1:0a 9.3 9.6

±

0.1 9.8 9.5 16 42

±

5 36W1:1a 7.0 8.4

±

0.8 9.6 9.5 16 48

±

9 71W1:3a 5.0 5.4

±

0.3 9.6 5.0 39 63

±

6 25W1:3aF 4.8 5.1

±

0.3 6.4 4.9 41 61

±

8 19W1:0bN 9.1 9.4

±

0.2 9.7 9.5 28 43

±

8 17W1:0b 9.1 9.6

±

0.2 9.7 9.6 22 39

±

6 40W1:1b 6.1 7.9

±

1.0 9.7 8.1 31 57

±

6 34W1:3b 5.4 6.8

±

1.2 8.5 7.2 38 62

±

8 36W1:3bF 5.5 6.9

±

0.8 7.7 7.5 37 65

±

5 30

MC = moisture content. For definitions of other symbols/abbreviations see footnotes to Table 2. For pH

mean

, n = 18, 27 and 10 in Experiments 1, 2 and 3. For MC

mean

, n = 8, 26 and 11 in Experiments 1, 2 and 3.

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Sanitation with respect to indicator microorganisms

In Experiment 1,

E. coli

was initially present in concen-trations of 10

3

–10

4

colony forming units (cfu.) per gram.No

E. coli

was detected (detection limit 10

2

cfu g

−

1

) onday 6 in any of the samples taken from any of the exper-

imental runs. The decreases in counts of total coliformsand

Enterococcus

spp. in material sampled from reactorsin Experiment 1 are shown in Table 4.

The decrease in

Enterococcus

spp. by day 12 was10

2

cfu g

−

1

in run V1:0, 103 cfu g−1 in run V1:1 and >104

cfu g−1 leading to no detection in run V1:3. There wasno detection (detection limit 102 cfu g−1) in Enterococ-cus spp. in samples taken on day 6, 12 and 18 in runV1:3 and after day 12 in run V1:1, while in run V1:0concentrations of 103 cfu g−1 were still detectable onday 43.

In Experiment 2, E. coli and total coliforms were notdetected in any sample taken from runs W1:0a andW1:1a on and after day 3, corresponding to a >102–103

reduction. In run W1:0a, the decrease in Enterococcusspp. was 103 cfu g−1 by day 6 and there was no detec-tion, corresponding to a reduction of >104 cfu g−1, byday 14. Enterococcus spp. was detected in concentra-tions of 103 cfu g−1 on day 14 in run W1:1a but was notdetected at the following sampling on day 40, corre-sponding to a reduction of >104 cfu g−1.

In Experiment 3, E. coli was detected at a stablelevel of >105 cfu g−1 in all samples taken from runW1:0bN up to day 40, while in run W1:0b no E. coliwas detected on and after day 6. Total coliformsbehaved similarly, with no reduction in W1:0bN and nodetection at day 6 in W1:0b. Enterococcus spp., initially

Table 3. Summary of temperature data recorded in compost reactors in Experiments 1, 2 and 3.

Exp. Run Tmax,av (°C)a (Tc–Tcorn,Av) (°C)b Tc–Ta (°C)c Timed, days

1e V1:0 60.9 0.8±1.8 18.6±6.9 4V1:1 67.9 0.3±1.6 31.1±5.0 >18V1:3 74.1 0.5±1.6 34.0±6.4 >18

2 W1:0aN 41.6 2.5±3.2 6.8±5.9 0W1:0a 60.8 1.9±2.4 18.9±8.3 6W1:1a 71.6 1.8±2.4 31.6±10.2 25W1:3a 41.0 0.8±1.2 2.8±5.8 0

W1:3aF 42.5 0.6±1.2 2.7±6.1 0

3 W1:0bN 49.3 2.6±2.8 9.5±7.3 0W1:0b 66.3 1.5±2.2 20±10.1 12W1:1b 38.1 0.2±1.1 4.0±3.2 0W1:3b 36.7 0.2±1.1 1.2±3.5 0

W1:3bF 39.0 0.3±0.9 1.9±3.6 0

aTmax,av (°C) = mean maximum temperature recorded during each measurement, where the maxima are for the five reactor points (centre and four corners).b(Tc–Tcorn,Av) = mean difference (over the measurement period), between centre temperature and mean corner temperature on each measurement occasion.c(Tc–Ta) = mean differences between centre temperature and ambient temperature over the measurement period.dTime in days is the time when the mean temperature for all five points exceeded 50 °C,eTemperature measurements in Experiment 1 were performed over a period of 18 days only. Temperatures were measured for over 40 days in Experiments 2 and 3.Ambient temperatures were minimum 24 °C, mean 29.7 °C and maximum 33 °C in Experiment 1; minimum 20 °C, mean 24.9 °C and maximum 32 °C in Experiment 2, and minimum 20 °C, mean 25.6 °C and maximum 34 °C in Experiment 3.

��

��

��

��

��

��

��

��

� � �� �� ��

�� ������

�

� �����

����

���� ����

���� ����

Figure 2. Temperatures in reactors V1:0, V1:1 and V1:3in Experiment 1 [mean, maximum and minimum of fivemeasurements taken on each occasion]. Amb. is ambienttemperature.

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492 C. Niwagaba et al.

present in concentrations of 106 cfu g−1 in runs W1:0bNand W1:0b, decreased by 102–103 cfu g−1 by day 18, but104 cfu g−1 in W1:0bN and 103 cfu g−1 in W1:0b couldstill be detected on day 40. Runs W1:0aN, W1:3a,

W1:3aF in Experiment 2 and runs W1:1b, W1:3b andW1:3bF in Experiment 3, which never heated up tosanitising temperatures, were not sampled for E. coli,total coliforms or Enterococcus spp.

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��

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��

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� � �� �� �� �� �� �� �� �� ��

�� ������

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��������

����

�����

�����

����

Figure 3. Temperatures in reactors reaching sanitising temperatures in Experiment 2 [mean, maximum and minimum of fivemeasurements taken on each occasion]. Amb. is ambient temperature.

��

��

��

��

��

��

��

��

� � �� �� �� �� �� �� �� �� ��

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Figure 4. Temperatures in a reactor reaching sanitising temperatures in Experiment 3 [mean, maximum and minimum of fivemeasurements taken on each occasion]. Amb. is ambient temperature.

Table 4. Bacterial counts (cfu g−1) of the two indicator organisms used in Experiment 1, total coliforms and Enterococcus spp.

Organism Run Day 0 Day 6 Day 12 Day 18 Day 43

Total coliforms V1:0 4.2×105 5.0×104 3.2×103 n.d. n.d.V1:1 2.4×105 9.0×103 n.d. n.d. n.d.V1:3 1.2×105 2.1×104 n.d. n.d. n.d.

Enterococcus spp. V1:0 3.7×106 2.2×104 1.1×104 1.0×103 1.0×103

V1:1 1.5×106 2.7×104 2.3×103 n.d. n.d.V1:3 2.1×106 n.d. n.d. n.d. –

n.d. = not detected, i.e. the counts were below detection limit of 102 cfu g−1.

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Discussion

The mixtures of source-separated faeces/ash and foodwaste were prepared by volume in Experiment 1because it is the easiest method that can be adoptedunder local conditions. Weighing scales may not alwaysbe available, whereas 5, 10, 20 and 50 L buckets arecommonly available at household and institution leveland can be used to mix faeces and food waste. However,during Experiment 1, it was noted that since thecompactness and/or density of the raw materials is neveruniform, the mix ratios prepared on a volume basis werenot easily replicable. Hence the mixtures were preparedaccording to weight in Experiments 2 and 3.

Because of the failure to reach sanitising tempera-tures in Experiment 2, the amounts of sawdust and oldcompost were increased in Experiment 3 with the aimof improving the conditioning of the substrates.

Temperature and insulation

In well-functioning composts, temperatures rise to ther-mophilic levels (>45 °C) in hours or a few days [7]. Thespeed of the temperature rise and its duration depend onmany factors, including available carbon and nitrogen,moisture, pH, oxygen concentration in the free airspace, and insulation [24,25]. In Experiment 1, temper-atures rose quickly in all reactors to >50°C, which wasmaintained continuously for more than two weeks inruns V1:1 and V1:3. However run V1:0, which did notcontain food waste, maintained >50°C for only aboutsix days (Figure 2). This agrees with earlier findingsthat suggest that the inclusion of food waste improvesthe composting of source-separated faeces at smallscale [5,9]. Food waste improves the compost mixtureby increasing easily degradable organics, which in thepresent experiments was reflected in higher fractions oforganics in the initial mixture, more degradation andlonger periods above 50°C (Tables 2 and 3, Figures 2and 3).

It proved possible to compost faeces/ash mixtureswith a high ash content (VSinitial = 23% in W1:0a,Table 1), but high temperatures were maintained for ashort time (less than one week) and the degradation insuch mixtures was only 30–40%. Faeces/ash and foodwaste mixtures, which were thermally composted,maintained high temperatures longer, e.g. in excess oftwo weeks, and the degradation of these mixtures was>60%. The high degradation obtained in the compost-ing of faeces/ash and food waste indicates a high activ-ity, resulting in more heat being released [20]. In thesestudies, the organic matter fraction of faeces/ashsubstrates was increased by removing some of the ashthrough sieving on a quarter-inch sieve, but this methodis unhygienic. Besides, any ash that sticks and dries onthe surface of the faeces is difficult to remove by siev-

ing. Therefore, a more appropriate alternative forachieving a high content of organics suitable forcomposting the faeces/ash mixture could be to replacesome of the ash during the collection phase with mate-rials containing higher organics, e.g. sawdust.

Non-insulated faecal/ash materials did not reachsanitising temperatures in Experiment 2 and did notsustain these temperatures for even one day in Experi-ment 3. In Experiments W1:0aN and W1:0bN thetemperature was on average 2.6 °C colder in the reactorcorners than in the centre, and the temperature in thecorners was on average 7–10 °C warmer than the ambi-ent air (Table 3). In runs in insulated reactors thatreached sanitising temperatures, the corners were onaverage only 0.3–2.0 °C colder than the centre, whilethey were 19–34 °C warmer than ambient air (Table 3).Previous studies have shown that composting in non-insulated reactors does not produce sanitising tempera-tures, as the heat is too easily lost to the surroundings[5,9,26]. Insulation helps to retain the heat within thematerial, decreases the relative volume of low tempera-ture zones [7,9,10] and ensures a near uniform temper-ature in the reactor (Table 3). Thereby, for propersanitisation without excessive mixing, the compostreactor should be insulated even when energy-rich foodwaste is added.

Influence of moisture content

The optimum moisture content (MC) for good compost-ing is between 40% and 60% [7,10,17,19,27]. Thedifferences in the optimum moisture content are mainlydue to differences in the structure of the substrates used.When the moisture content becomes too high it affectsparticle aggregation, matrix air-filled porosity and thusmatrix gas permeability, limiting the transport of theoxygen necessary for the composting process [10,17],and this is the probable reason why most of thecomposts containing food wastes and initial MC >65%in Experiments 2 and 3 did not reach sanitising temper-atures (Tables 1 and 3). There appeared to be a break-point in moisture content at around 60–65%, beyondwhich the moisture content impeded the compostingprocess (Figure 5). Run W1:1a was close to this break-point (initial moisture content = 63%) and this probablycaused the long delay in reaching sanitising tempera-tures in this run (Figure 3). The moisture content of runW1:1a decreased slowly by evaporation from 63% onday 1 to about 50% on day 19, when the temperatureincreased and remained well over 50 °C for more thanthree weeks. In Experiment 3, the run with initial mois-ture content as low as 43% (W1:0b) achieved a fast risein temperature, while runs with moisture content greaterthan 65% (W1:1b, W1:3b and W1:3bF) did not attainsanitising temperatures.

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Figure 5. Mean temperature maxima at five measuring points in the compost reactors plotted against initial moisture content of the substrate.

The higher the initial moisture content, the lower thepH during composting (Figure 6) indicating organicacid formation and accumulation under anaerobicconditions [15,20], especially at moisture content above65%. The five runs with a moisture content >65% had a

minimum pH of 6.1 or lower and none of these reachedtemperatures above 43 °C (Figures 5 and 6), whichagrees well with the inhibition of thermophiliccomposting at pH <6.5 [16]. In Experiment 2, the runsthat heated to sanitising levels had a pH in the region of

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Figure 5. Mean temperature maxima at five measuring points in the compost reactors plotted against initial moisture content ofthe substrate.

4

5

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8

9

10

40 50 60 70 80

Initial MC (%)

pHm

in

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Figure 6. Minimum pH during composting plotted against the initial moisture content of the substrate.

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Environmental Technology 495

8.5 to 9.7, either from the start (W1:0a), or increasing tothis range with time as the temperature increased (runW1:1a). The pH of materials in runs that never heatedto thermophilic conditions, due to high water content,continued to decline to about 5.0 in runs W1:3a andW1:3aF.Figure 6. Minimum pH during composting plotted against the initial moisture content of the substrate.Thus it is better to start the compost with drysubstrates and add water when needed, rather than tostart with too high a moisture content that may inhibitthe compost process. The use of the fist test or visualinspection of the starting compost, to determinewhether it has the approximate correct moisture toinitiate the process, is inaccurate, particularly for freshcomposts with a high proportion of food waste, mainlybecause the initially intact cells of the food wastecontain much water that is hard to detect by hand orvisually. This was experienced in runs W1:3a, W1:3aF,W1:1b, W1:3b and W1:3bF which, when the kitchenwaste degraded, transformed from a well-structuredsubstrate into a waterlogged paste that could not ther-mally compost. In runs W1:1b and W1:3b, the wetmaterials were removed from the compost boxes andair-dried for 1–2 days, decreasing the moisture contentfrom >65% to 50%. However, this did not result in anysubsequent increase in temperature, possibly due toinhibition by the low pH [15]. Thus, if the substrate isfound to be too wet, the common recommendation toadd dry material seems to be preferable to drying thesubstrate.

In Experiments 1, 2 and 3, materials with startingpH >7.0 and even as high as 9.8 in insulated reactorscomposted well, attaining thermophilic temperatures.Thus, for this ash/faeces/food waste substrate, a well-functioning composting process was attained up to apH of almost 10.0, which is above the pH range (6.5–9.0) normally recommended [9,15,16,17]. Possibleexplanations for the functioning of the compostprocess at such high pH values in this study can be thatthe substrate was non-homogeneous, with regions oflower pH. In addition, pH <10.5 is not lethal tocomposting bacteria, and thus it is possible that thecomposting process can proceed even in regions ofhigh pH [7].

Sanitation with respect to indicator microorganisms

When the temperature reached above 50 °C, no E. colior total coliforms were detected within three to sixdays. This is in line with an earlier study in whichcomposting dairy manure at 55 °C decreased E. coli ,(Listeria and Salmonella spp.) to below detectionwithin three days [28].

Enterococcus spp. was not detected in the reactorsthat maintained temperatures above 50 °C for more thantwo weeks. The higher the temperature margin above 50

°C and the longer the time that high temperatures inexcess of 50 °C were maintained, the shorter was thetime to no detection. Enterococcus spp. was detectedthroughout the study in runs V1:0, and W1:0b evenwhen temperatures had been well above 50°C for >12days. However, the detection could also be due to non-homogeneous survival in low temperature zones, asmixing was done with a spade, mainly in the horizontaldirection, even though some vertical mixing wasachieved. Therefore, complete mixing of the substrateswas not guaranteed. Two areas maintained lowertemperatures than the rest of the reactor: an area aroundthe incoming ventilation and the upper volume near thesurface. Proper mixing to ensure that all material isheated to temperatures >50 °C for at least one weekduring composting is recommended to ensure that allmaterial is safely sanitized [2]. This requires that allcompost particles reach and maintain >50 °C for aperiod of at least one week, which is difficult to guaran-tee. The hygiene safety can be increased by a longertime at temperatures above 50 °C.

In studies in a smaller (78 L) reactor insulated by25 mm styrofoam, Enterococcus spp. was reduced tobelow detection at day 11 after eight days above 50 °C[9]. In the 216 L reactors in this study, where mixing –which was mainly horizontal – was more difficult, sani-tising temperatures were maintained for 12 days.However, it was still possible to detect high levels ofEnterococcus spp. in samples from some runs even aftermore than one month of composting.

Conclusions

Source-separated faeces/ash mixtures with VS contentas low as 23% of TS had enough energy to thermallycompost in a 216 L reactor, provided the reactorwalls were insulated to decrease the heat loss. Source-separated faeces/ash with high pH, close to 10.0, werethermally composted. However, one setback withcomposting just source-separated faeces/ash mixturewas that sanitising temperatures were maintained for ashorter time, only 12 days or less, and thus the mate-rial was not sanitized.

This study shows that it is possible to maintain sani-tising temperatures for longer (more than two weeks)when composting mixtures of source-separated faeces/ash and food waste at the 216 L scale, provided that thereactor is insulated by 75 mm styrofoam. However,even if high temperatures are reached, this is not a guar-antee that intestinal pathogenic microorganisms havebeen inactivated, especially where complete mixing isnot guaranteed. Thorough mixing to ensure all materialis exposed to high temperatures, and perhaps alsoprolonged periods at high temperature, are needed toproduce sufficiently hygienic compost.

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The fist test and/or physical inspection to estimatethe moisture content of starting substrates containingfood waste is inaccurate. Food waste may contain largequantities of water in intact cells, which upon disinte-gration result in loss of structure, producing a water-logged material that does not thermally compost. In thisstudy, air-drying to decrease moisture did not helprestore the composting process because of inhibitiondue to low pH. Therefore, it is better to start the processsomewhat drier and add water when needed rather thanto risk starting with substrates that are too wet. Forinitially wet substrates, addition of drier substrates formoisture correction should be investigated.

AcknowledgementsThe authors express their gratitude to Sida/SAREC for finan-cial support. The authors also thank the staff of the Micro-biology Laboratory at the Faculty of Veterinary Medicine,Soil Science Laboratory, Faculty of Agriculture, PublicHealth/Environmental Engineering Laboratory and Faculty ofTechnology at Makerere University, Kampala, Uganda, whoassisted with analyses. Dauda Kiwanuka is thanked for fabri-cating the compost reactors, and assisting in sampling andanalysis. We thank Mary McAfee for checking the Englishlanguage in this paper.

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