4 Case 4: Composting of Selected Organic Sludges Using Rotary Drum

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polystyrene to maintain the temperature and was operated in a continuous rotation with a

sufficient air supply (40).

The pH of composting sludge mixtures was between 5.39 and 7.75, and the moisture

content for the organic sludges was more than 80%, except for leachate sludge where the

moisture was 63.9%. The C/N ratio of these raw organic sludges was low, ranging from 7 to

19. The food factory sludge contained a high total microbial count at 2.7 × 1010 cfu/g and

followed by POME sludge, sewage sludge, and leachate sludge at 1.0 × 108 , 2.0 × 107 , and

7.0 × 106 cfu/g, respectively. The nutrient concentration of P, K, and Mg was high in POME

sludge, which measured at approximately 12,602, 2,118, and 322 ppm, respectively. However,

the highest concentration of Mn was found in sewage sludge, measured at 606 ppm. Overall,

sewage sources contained the highest concentration of heavy metals in raw sludge (40).

The composting rate was also studied using a mixed ratio of 3:1 (sludge to bulking agent).

In the study, it appeared that sludge from sewage, POME, food factory, and leachate underwent

the fermentation phase of approximately 5, 5, 10, and 13 days respectively, while the curing

took about 35, 30, 30, and 17 days, respectively, to achieve completion. In the final stage,

decomposition rate measured was recorded to be about 60, 52, 55, and 50% for sewage,

POME, food factory, and leachate sludge, respectively. The best achievement for composting

of sewage sludge, POME sludge, food factory sludge, and leachate sludge were approximately

40, 35, 40, and 30 days, respectively (40).

Leachate sludge compost product measured the highest pH of 8.03. In terms of P, K, and

Mg, the highest value was found in POME sludge compost, while the highest Ca and Mn were



Fig. 5.6. Schematic diagram of rotary drum composter (40).



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found in leachate sludge compost products. In comparison, the concentration of heavy metals

in final compost products decreased in the following order: POME sludge > sewage sludge >

food factory sludge and leachate sludge. In terms of physical characteristics, the research

compost products were dark brown and had an earthy smell. The number of total coliform

bacteria was recorded to be less than 102 cfu/g. Using a growth study, the germination rates

using compost from POME sludge, sewage sludge, food factory sludge, and leachate sludge

were 80, 90, 78, and 94%, respectively (40).

The compost product obtained in this study was applied as a biofertilizer for growing

spinach. Results showed that spinach grown with sewage sludge compost produced leaves

with a greener color in leaves and promoted superior growth (shown as Pot C in Fig. 5.7).

Continuous growth studies after 5 weeks indicated sustained greening of the leaves and good

growth especially for sewage compost (shown as Pot A in Fig. 5.8). In conclusion, it appears

that sewage sludge compost, food factory sludge compost, leachate sludge compost, and

POME sludge compost all showed similar characteristics as commercial composts (40).

In another study, the compost produced using a windrow system (heap method) and a rotary

drum system (composter) were compared. In the windrow system, composting was performed

using different percentage of inoculum with 0.1 and 1.0% Effective Microorganisms (EM)

(40). For both systems, pH values were around 6.58–6.85, and moisture content was around

65–67%. An important parameter for the composting process that needed attention was the



A



B



C



D



E



F



A – Growth using leachate sludge compost (LSC)

B – Growth using food factory sludge compost (FFSC)

C – Growth using sewage sludge compost (SSC)

D – Growth using commercial compost (CC)

E – Growth using POME sludge compost (PSC)

F – Control



Fig. 5.7. Growth of spinach in different compost products after 3 weeks.



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A



B



C



D



E



F



A – Growth using sewage sludge compost (SSC)

B – Growth using commercial compost (CC)

C – Growth using leachate sludge compost (LSC)

D – Growth using POME sludge compost (PSC)

E – Growth using food factory sludge compost (FFSC)

F – Control

(Note: arrangement of pots followed the order of growth)



Fig. 5.8. Growth of spinach in different compost products after 5 weeks.



C/N ratio. For a windrow system, the C/N ratio started around of 22–24, while for the

rotary drum system, the C/N ratio started at 28 because of the low nitrogen content in mixed

substrates compared to the nitrogen value in windrow system, which is more than 2%. The

total microbial count for windrow (0.1%EM), windrow (10%EM), and rotary drum were

around 1.1 × 107 , 1.0 × 108 , and 8.6 × 108 cfu/g, respectively. The highest number of total

coliform bacteria was obtained from windrow system (0.1%), measured at about 1 × 106 .

The nutrient content presented a higher value in the rotary drum composter, especially

for P, K, Ca, and Mn; the data recorded were 1382, 873, 1011, and 80 ppm respectively.

Similarly, the concentration of heavy metals for Fe, Zn, Pb, and Ni were found to be the highest

for the rotary drum system; the values were 2547, 107, 41, and 329 ppm, respectively (40).

Generally, the physical, chemical, and biological characteristic showed that compost products

were similar to those of the commercial composts. In terms of the number of pathogens and

the concentration of heavy metals, they all complied with the standards of USEPA and were

suitable for use as biofertilizer and soil conditioner.



8.5. Case 5: Bioreactor Co-composting of Sewage Sludge and Restaurant Waste

Three different types of dewatered sewage sludge, i.e., septic tank, oxidation pond, and

activated sewage sludge were co-composted with municipal solid waste in a two-stage process.



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Fig. 5.9. Section view of 200 L – bioreactor composter (41).



The first phase of the co-composting process, known as the fermentation phase of sewage

sludge and restaurant waste, was performed in a 200-L bioreactor (Fig. 5.9). Shredded garden

waste was added as bulking agent. A 2:1 (wt/wt) ratio of municipal solid waste and sewage

sludge was found to give the best initial C/N ratio for the composting process. The second

phase of composting process was performed in an open space using a windrow system (heap

method). The produced compost was characterized and the results were almost identical to

commercial compost and also complied with US EPA standards (41).

A growth study using produced compost to grow spinach showed satisfactory results. The

ratio of the compost to the soil was 2:1 based on a volume basis. It was found that the

growth of spinach using compost produced from the oxidation pond and activated sewage

sludge was almost identical to that of commercial compost (Fig. 5.10). The spinach that

grew in the activated sewage sludge compost product produced more greenish color in the

leaves (36).



NOMENCLATURE

C = carbon

Ca = calcium

CC = commercial compost

Cd = cadmium

cfu = colony forming units (coliform count)

cm = centimeter

CO2 = carbon dioxide

Cr = chromium