Waste Management 30 (2010) 977–982

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Waste Management

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Microbial inoculants for small scale composting of putrescible kitchen wastes

J. Nair *, K. OkamitsuEnvironmental Technology Centre, Murdoch University, Perth, Western Australia 6150, Australia

a r t i c l e i n f o

Article history:Accepted 6 February 2010Available online 6 March 2010

0956-053X/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.wasman.2010.02.016

* Corresponding author. Tel.: +61 8 9360 7322; faxE-mail address: j.nair@murdoch.edu.au (J. Nair).

a b s t r a c t

This research looked at the need for ligno-cellulolytic inoculants (EM bacteria and Trichoderma sp.) insmall to medium scale composting of household wastes. A mixture of household organic waste com-prised of kitchen waste, paper, grass clippings and composted material was subjected to various dura-tions of thermo composting followed by vermicomposting with and without microbial inoculants for atotal of 28 days. The results revealed that ligno-celluloytic inoculants are not essential to speed up theprocess of composting for onsite small scale household organic waste treatment as no significant differ-ence was observed between the control and those inoculated with Trichoderma and EM in terms of C:Nratio of the final product. However, it was observed that EM inoculation enhanced reproductive rate ofearthworms, and so probably created the best environment for vermicomposting, in all treatment groups.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Onsite small scale composting of organic waste is encouraged asan effective solution to reduce the waste management problemglobally and several processes and techniques for effective com-posting have been discussed in the literature. The duration of thecomposting process to produce good quality end product varieson the type of waste and size of the system. Different processesare adopted to accelerate composting processes and maturationof the product.

The ligno-cellulose content of waste particularly of biomasssuch as wood and straw which has approximately 40% cellulose,20–30% hemicelluloses and lignin (Sjostrom, 1993) is difficult tobreakdown in a normal composting process and can take consider-able period of time. Inoculation with bacteria and fungi which canbreakdown ligno-cellulolytic material has been reported to beeffective in composting (Tiquia et al., 1997; Bolta et al., 2003).For example inoculation of complex microorganisms such as Bacil-lus casei, Lactobacillus buchnei and Candida rugopelliculosa and lig-no-cellulolytic fungi (Trichoderma sp. and white-rot fungi)accelerate humification and maturation in composting process(Wei et al., 2007). Bacteria that are the dominant in the process in-clude Bacillus species. It was reported that 87% of bacteria in ther-mophilic composts is of genus Bacillus (Strom, 1985b). Thermus sp.can tolerate as high temperatures as 65–82 �C (Beffa et al., 1996).Actinomycetes are also thermophilic bacteria with genus Nocardia,Streptomyces, Thermoactinomyces and Micromonospora as represen-tative forms isolated from compost (Waksman et al., 1939; Strom,1985a), and identified as important agents of ligno-cellulose degra-

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dation (Crawford, 1983; Godden et al., 1992). Trichoderma sp. is acommon fungi in soil (Leandro et al., 2007) and known to degradehemicelluloses, facilitating compost stabilization and acceleratecomposting process of the waste material (Singh and Sharma,2002; Pramanik et al., 2007, 2009).

The best way to combine high-temperature microbial compost-ing (called ‘thermo composting’ in this paper) with vermicompo-sting has been a subject of recent research (Nair et al., 2006). Nowork has been conducted on how inoculants could help or hindera combined thermo/vermicomposting system. It was consideredimportant to understand whether inoculation of bacteria or fungiis essential for treatment of normal household wastes and/orwhether inoculation will improve the quality of a vermicompostproduct in terms of stability and maturity. The objective of thisstudy is therefore to test the impact of bacterial inoculum, EMand a fungal inoculum, Trichoderma sp. on the thermo and vermi-composting of household organic waste.

2. Materials and methods

2.1. Methodology

The waste materials for the premix for composting were se-lected based on the generation from domestic sources and consid-ering the need for sufficient quantities of bulking agent to treatputrescible kitchen waste. It has been observed that compost canbe a good starting material for a fresh batch of composting process.Therefore in this experiment, the premix contained compost from aprevious trial using kitchen waste. The compost premix was pre-pared by mixing compost (30%), sawdust (16%) paper/cardboard(4%), grass clippings (20%) and kitchen waste (30%). Paper/card-board included shredded office paper, card board, and news paper

978 J. Nair, K. Okamitsu / Waste Management 30 (2010) 977–982

and kitchen waste, consisted of vegetable scraps, coffee grinds andfood leftovers. Therefore it can be considered that for composting a30% putrescible waste, a total of 70% of other ingredients wereused as bulking agents and as carbon sources. The control is thepremix by itself and the tests were the premix with EM and Trich-oderma sp. inoculated. Each waste component was tested for pH,conductivity, moisture content, total carbon and total nitrogen(TKN).

2.2. Inoculants used

A commercially available microbial seeding agent known as EM,procured from VRM Pty Ltd., Australia (www.vrm.com.au), is amixture of different bacterial species made for accelerating thecomposting process. EM culture was prepared according to themanufacturer’s instruction. The 500 ml culture was mixed with500 ml sterile distilled water. The Trichoderma sp. culture was pre-pared in potato dextrose agar and the inoculation material wasprepared from the pure culture. The 1 l inoculum was then sprin-kled onto the material in the compost barrel, rolling regularly foreven mixing. The inoculum was added on days 7, 14 and 21. After28 days, samples from all trials were tested for the total bacterialcount (CFU/g) using a pour plate method with nutrient agar andfungal count was tested using the potato dextrose agar.

2.3. Thermo composting and vermicomposting sequence

The two tests conducted compared: (1) composting efficiency ofthe control with the bacterial and fungal inoculated systems and(2) complete thermo composting (TC) with the combination ofthermo composting and vermicomposting (VC). The dual process(TC + VC) is a 7–21 days of TC followed by the remainder of the28-day period of VC. The total treatment period in all the trialswas 28 days as it has been found (Nair et al., 2006) that the com-bination of TC + VC can produce a stable compost by 21 days. Ther-mo composting was conducted in compost barrels of 230 l capacityusing the compost premix. Three trials (control and 2 inoculants)were conducted in triplicate. Inoculations were done on the firstday of the experiment and then every 7, 14 and 21st day in the testbarrels. The barrels were rolled 10� every day for mixing and aer-ation. The moisture content was maintained between 53% and 61%throughout the experiment in the TC barrels. Reducing the mois-ture content was necessary when having putrescible kitchen wastein the premix and in this experiment, around 70% of bulking agents(sawdust, compost, lawn clippings, paper and cardboard) wereused to reduce the high moisture content of kitchen waste. Thetemperature, pH, conductivity and moisture content were mea-sured daily after rolling the barrels. Moisture content was mea-sured by drying samples in the oven at 105 �C for 48 h and otherparameters using HACH portable probes of the supernatant ofthe sample made after mixing 1 g of sample in 10 ml of distilledwater.

For VC set ups where parallel TC and VC trials were conductedfor a total of 28 days starting from the same premix of ingredients,on every seventh day, 2 l of materials from the barrels were trans-ferred to 2 l containers with perforated lids, in which 10 red worms(L. rubellus) were released for further processing of the partiallycomposted material. Bed height for volume reduction, pH, and to-tal number of adult worms, juveniles and cocoons in each con-tainer were counted weekly for 28 days.

2.4. Chemical analyses

Weekly samples were collected from the TC barrels and VC con-tainers for testing pH, total carbon and total Kjehldahl nitrogen. To-tal carbon was tested using high temperature non-dispersive

infrared gas analyser and total nitrogen as per APHA (2005). Atthe end of the experiment final samples were analyzed for totalcarbon, total Kjehldahl nitrogen, nitrate as well as total phosphorusand orthophosphate using an autoanalyser.

2.5. .Statistical analyses

One-way ANOVA (SPSS ver. 15.0) was used to find out whetherthere was a significant difference (p < 0.05) between the treatmentgroups. For those normally distributed data sets, LSD was used, andfor not normally distributed data sets Tamhane’s T2 was used.

3. Results and discussion

All three trials (control, EM inoculated and Trichoderma inocu-lated) showed similar changes in temperature and pH throughoutthe composting period (Figs. 1 and 2). The temperature peakedover 50 �C in the first 7 days and remained between 30–35 �C fornext 2 weeks. By the 23rd day it dropped to 25 �C which is consid-ered to have entered the maturation phase. The pH decreased inthe early stage to a slightly acidic state and towards the 20th dayit gradually increased to a neutral pH and was almost neutral inall treatment groups in all composting schedules. This is in accor-dance with the pH changes that occur in a composting system, ini-tial drop followed by pH stabilization. The process being (1) themineralization of nitrogen such as nitrates, nitrites, and other or-ganic acids, (2) the microorganisms convert organic matter, intoCO2 and humic substances releasing heat, (3) the degradation ofsoluble and easily degradable carbon sources, such as monosaccha-ride, starch and lipids increases organic acids of the material. In thenext stage proteins are degraded to ammonium and would resultin an increase in pH (Cáceres et al., 2006; Paatero et al., 1984;Ndegwa and Thompson, 2001).

After the initial breakdown, larger organic compounds, such asligno-cellulose, are degraded to humic substances (Crawford,1983; Paatero et al., 1984) and this is when the inoculants were ex-pected to play their role. During this stage, the waste mixtureexperiences mass reduction, stabilization and pathogen reduction.It is also understood that compost needs to be kept at 55 �C for15 days to maximize composting efficiency and pathogen reduc-tion. In this study, the thermophilic phase lasted for only a week,with the maximum temperature of 55 �C retained only for 2 days(treatments with Trichoderma did not reach 55 �C). This is commonin systems that use smaller volume as in this experiment where itis susceptible to heat loss due to the high surface–volume ratio(Nair et al., 2006).

Electrical conductivity increased constantly from around 400 to1200 lS/cm during the first 12 days which could be due to releaseof mineral salts and ammonium ions through the decomposition oforganic matter followed by a general drop to around 600 lS/cm(Fig. 3) which could be attributed to the precipitation of mineralsalts and volatilization of ammonia (Huang et al., 2006). Volumeof compost (l) decreased during the experiment and reached themean volumes between 54% and 56% of the initial volume in thethree treatments (Fig. 4).

Inoculations did not augment TC process as all the physical andchemical parameters tested were similar to the control having noinoculums. There was no difference noted in volume reduction,pH, temperature, total carbon content and TKN in the inoculatedsystems compared to the control. From Table 1 it can be seen thatthe TKN of the initial ingredients was low which could have af-fected the TKN of the final product. The initial C:N ratio of 79.1was reduced to <35 in the first week of composting as shown inFig. 7. This could be due to carbon loss which can be linked to vol-ume reduction that occurred in the first 7 days (Fig. 4). After

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Fig. 1. The changes in pH in compost barrels (bars show standard deviations, n = 3).

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Fig. 2. Temperature variation in compost barrels.

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)

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EM

Trichoderma

Fig. 3. Change of electrical conductivity in compost barrels (lS ± SD).

J. Nair, K. Okamitsu / Waste Management 30 (2010) 977–982 979

4 weeks of composting, the ratio reduced to around 23, 27 and 23in the control, EM inoculation and Trichoderma sp. inoculationgroup respectively. The overall high C:N ratio (>23:1) is attributedto the unchanged TKN during the composting process.

Previous studies have shown contrasting results when usingbioinoculants in the composting process (Golueke et al., 1954;Nakasaki and Akiyama, 1988; Faure and Deschamps, 1991; Elorri-eta et al., 2002; Gaind et al., 2005); however as Vargas-Garcia

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Day1 Day7 Day14 Day21 Day28

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me(

L)

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Fig. 4. Changes in volume of material after TC barrels (L ± SD).

02468101214161820

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14TC

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28TC

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(mg/

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Fig. 6. TKN of the in the final sample of TC and VC substrate (mg/g ± SD).

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Fig. 5. Total carbon in the final sample of TC and VC substrate (% ± SD).

980 J. Nair, K. Okamitsu / Waste Management 30 (2010) 977–982

(2006) suggested, the activity of various inoculants will vary withthe raw material. It seems that for composting kitchen waste, theinoculants such as EM and Trichoderma sp. are not very effective.This could also be because the premix had mature compost (30%)that normally harbors microbes that will facilitate the compostingprocess.

The addition of inoculants in sequential TC + VC seemed toaccelerate carbon degradation. Most effective reduction of totalcarbon was observed in 21 days TC followed by 7 days VC withTrichoderma inoculation. TKN was expected to increase duringthe VC process as Garg et al. (2006) reported an increase of 4.4-to 5.8-fold in 100 days. However, in the present study there wasno obvious increase in TKN (Fig. 6). TKN content is primarilydependent on the initial total nitrogen content of the organic wasteand the rate of decomposition of the waste (Crawford, 1983; Shar-ma, 2003). Lowest C:N ratio in this study was observed in21TC + 7VC inoculated with Trichoderma followed by EM whichcorresponds to the total carbon reduction in those trials (Fig. 5).Morais and Queda (2003) suggested a ratio under 20 is optimalfor maturity and the best ratio is lower than 15. Nair et al.(2006) obtained a C:N ratio less than 20 in 21 days by combiningTC and VC. However in the present study only those inoculatedwith Trichoderma sp and having 21 days thermo composting plus7 days vermicomposting reduced the ratio to <20. However therewas no significant difference between the treatment groups(p > 0.05) (Fig. 7).

Nitrate nitrogen of the final product was greater with TC + VCthan TC by itself (Fig. 8). Tognetti et al. (2007) reported that nitratecontent increased constantly with composting and vermicompo-sting of municipal organic waste for 130 days. Organic N is con-verted to ammonia in the thermophilic phase of compostingprocess, which raises pH and towards the end of thermophilicphase, the converted ammonia, which is unstable, is further con-verted to more stable nitrite and nitrate through nitrification bymicroorganisms (Bernal et al., 1998). Earthworms contribute to in-creased microbial activity by breaking down and aerating the sub-strate (Nair et al., 2006). In this study, although no statisticallysignificant difference between groups (p > 0.05) was observed, con-trol trials showed constant increase in proportion with vermicom-posting duration from 0.12 to 0.45 mg/g, and Trichoderma

Table 1Total carbon, TKN and C:N ratio of each initial component used for the experiment.

Final mix Grass clippings Com

Total carbon (%) 39 27TKN (mg/g) 24 23C:N ratio 76.1 16.25 11.74

inoculation also showed increase in nitrate nitrogen with the in-crease in the period of vermicomposting. This explains better min-eralization of nitrogen with vermicomposting. In contrast, EMinoculation reduced the formation of nitrate (Fig. 8).

Total phosphorus (TP) was highest in 28 days TC inoculatedwith Trichoderma (Fig. 9). There was no significant effect onincreasing TP by adding inoculants even for the VC process. One-way ANOVA indicated significant difference for EM inoculated withother groups in the trial 7TC + 21VC (p < 0.05). During the com-posting process, release of organic acids to the substrate helps sol-ubilisation of insoluble phosphorus (Pramanik et al., 2007) andthrough worm activities further increase of extractable phosphatecan be achieved especially due to the enhanced microbial activityin VC process (Tognetti et al., 2005; Zhang et al., 2000). Microbiotain the gut of earthworms also contributes to increase of P (Sharma,2003; Tognetti et al., 2005; Zhang et al., 2000). However there wasno obvious trend indicating vermicomposting increases orthophos-phate as concluded by others (Nair et al., 2006). Immobilization ofnutrients due to large amount of C-rich bulking agents may have

post Sawdust Food scraps Paper/cardboard

51 46 411.6 45 0.9318.75 10.22 455.56

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Fig. 10. Orthophosphate in the final sample of TC and VC substrate (mg/g ± SD).

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Trichoderma

Fig. 8. Nitrate and Nitrite in the final sample of TC and VC substrate (mg/g ± SD).

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rus(

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ControlEMTrichoderma

Fig. 9. Total phosphorus in the final sample of TC and VC substrate (mg/g ± SD). Starindicating significant difference (p < 0.05) from other two trials.

Table 2Final bacterial and fungal population in inoculated and control thermo compostsamples after 28 days.

Total bacterialcount CFU/g

Total fungalcount CFU/g

Trichoderma inoculated 5.6 � 108 1,661,100EM inoculated 2.1 � 109 1,988,900Control 2.4 � 109 2,100,000

J. Nair, K. Okamitsu / Waste Management 30 (2010) 977–982 981

occurred resulting in slowing down solubilisation of P to extract-able phosphate and subsequent mineralization of P by earth-worms. Increasing the duration of vermicomposting may resultin an increase in extractable P (Fig. 10).

The impact of inoculants on worm reproduction was assessedthrough the number of cocoons per adult worm (total of 30 adult

worms per trial) is shown in Fig. 11. Average cocoon numbersper worm based on 30 adult worms was highest in EM inoculatedmaterial followed by Trichoderma inoculation and the reproductiverate was found to increase proportionally with the duration of ver-micomposting. The reason of high reproductive rates could be dueto better microbial activity and (Bajsa et al., 2003) a better sub-strate with bacteria/fungi for the worms to reproduce. In generalthe number of earthworms in a system was found to be inverselyproportional to C:N ratio (Ndegwa and Thompson, 2001; Airaet al., 2006). However if the bacterial and the fungal populationof the final product is considered, which is an indication of matu-rity of the compost were tested from all the trials (Table 2), allsamples developed almost equal amounts of bacteria and fungalpopulation in spite of inoculation of bacteria and fungus in the pro-cess of some trials. This could be due to the mature compost usedin the control which might have provided the essential microbes tostart in the composting process.

982 J. Nair, K. Okamitsu / Waste Management 30 (2010) 977–982

4. Conclusion

The study reveals that ligno-cellulolytic fungal or bacterial inoc-ulation is not necessary for thermo composting in small scale com-posting systems as the control without any inoculums performedequally well in relation to all parameters. However inoculationdid help vermicomposting process slightly as they provided a moreconducive environment or substrate for worm activity, althoughthe difference was not statistically significant between the inocula-tion trials and the control or between the inoculated trials. Thisstudy has shown that when mature compost is used in the premix,the composting process will perform equally well as inoculatingwith bacterial or fungal inoculums. It is suggested that to knowthe suitability of different inoculants to the composting processof a specific substrate and to improve their composting processprior investigation may be necessary. Therefore negative or no ef-fect noticed in this study will be worthwhile information to theknowledge in this area which is scarce and contradictory.

Acknowledgement

The authors thank Waste Management Board, Department ofEnvironment and Conservation, Western Australia, for the fundingthat supported this study.

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