Waste Management 29 (2009) 180–185

Contents lists available at ScienceDirect

Waste Management

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

Composting clam processing wastes in a laboratory- and pilot-scalein-vessel system

Zhenhu Hu a, Robert Lane b, Zhiyou Wen a,*

a Department of Biological Systems Engineering, Virginia Polytechnic Institute and State University, Virginia Tech, Blacksburg, VA 24061, USAb Virginia Seafood Agricultural Research and Extension Center, Hampton Road, VA 23669, USA

a r t i c l e i n f o

Article history:Accepted 21 February 2008Available online 10 April 2008

0956-053X/$ - see front matter � 2008 Elsevier Ltd.doi:10.1016/j.wasman.2008.02.016

* Corresponding author. Tel.: +1 540 231 9356; faxE-mail address: wenz@vt.edu (Z. Wen).

a b s t r a c t

Waste materials from the clam processing industry (offal, shells) have several special characteristics suchas a high salinity level, a high nitrogen content, and a low C/N ratio. The traditional disposal of clam wastethrough landfilling is facing the challenges of limited land available, increasing tipping fees, and strictenvironmental and regulatory scrutiny. The aim of this work is to investigate the performance of in-ves-sel composting as an alternative for landfill application of these materials. Experiments were performedin both laboratory-scale (5 L) and pilot-scale (120 L) reactors, with woodchips as the bulking agent. In thelaboratory-scale composting test, the clam waste and woodchips were mixed in ratios from 1:0.5 to 1:3(w/w, wet weight). The high ratios resulted in a better temperature performance, a higher electrical con-ductivity, and a higher ash content than the low-ratio composting. The C/N ratio of the composts was inthe range of 9:1–18:1. In the pilot-scale composting test, a 1:1 ratio of clam waste to woodchips wasused. The temperature profile during the composting process met the US Environmental ProtectionAgency sanitary requirement. The final cured compost had a C/N ratio of 14.6, with an ash content of167.0 ± 14.1 g/kg dry matter. In addition to the major nutrients (carbon, nitrogen, calcium, magnesium,phosphorus, potassium, sulfur, and sodium), the compost also contained trace amounts of zinc, manga-nese, copper, and boron, indicating that the material can be used as a good resource for plant nutrients.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

The clam industry in Virginia (USA) generates large quantities ofprocessing waste (shell, offal, and processing wastewater). Environ-mentally friendly disposal of these waste materials is a significantchallenge to the clam processing industry. Currently, clam wastedisposal is predominantly through landfilling. This managementpractice is under environmental and regulatory scrutiny due tothe limited amount of land available for disposal, and to increasedenvironmental concerns such as groundwater contamination andodor problems (Nicolas et al., 2006; Papadopoulou et al., 2007).

Composting provides an economically and environmentallysound alternative for landfill operations for clam waste disposal. Un-like landfilling that is limited by land and causes severe environmen-tal problems, composting converts organic wastes into a productthat can be blended into fertilizer or a soil amendment (Raviv,2005). As a waste management practice, composting has beenwidely used in the treatment of various waste materials includinganimal waste, fish waste, sewage sludge, and priority pollutants (Ad-ler and Sikora, 2005; Antizar-Ladislao et al., 2005; Hue and Sob-

All rights reserved.

: +1 540 231 3199.

ieszczyk, 1999; Margesin et al., 2006; Zubillaga and Lavado, 2006).The complex organic matters in these materials are decomposedand humified through various biological processes; bulking agents(e.g., woodchips) are usually added to improve composting perfor-mance – such as absorbing odor, adjusting the appropriate carbon-to-nitrogen (C/N) ratio, keeping the moisture constant, and enhanc-ing the porosity and aeration (Laos et al., 2002; Nicholls et al., 2002;Ros et al., 2006) Depending on the different compositions of thewaste materials, the composting performance may be quite differ-ent. For example, composting of a nitrogen/protein-rich (a low C/Nratio) pig sludge resulted in a rapid increase of temperature to ther-mophilic phase within 5 days (Tiquia and Tam, 2000); while in thecomposting a lignocellulose-rich (high C/N ratio) solid olive-millbyproduct (alperujo), the process exhibited an initial long meso-philic period (�8 weeks) before reaching thermophilic phase (Albur-querque et al., 2006). This temperature profile difference was due tothe fact that the nitrogen/proteins contained in pig sludge are morereadily degradable than the lignocellulosics contained in alperujo.

Compared with other waste streams, clam waste contains alarge amount of readily digestible protein and, thus, has a highcontent of nitrogen and a low C/N ratio. The material also has ahigh salinity due to the marine environment in which clam grows,and a high ash content due to the high content of shell residues.

Z. Hu et al. / Waste Management 29 (2009) 180–185 181

When clam waste is composted, these special features may resultin a unique composting performance and compost properties thatare different from other waste materials. The aim of this work is toevaluate the clam waste composting performance such as temper-ature profiles, effects of high salinity on compost, and nutrientslosses. The matured compost was also evaluated with regard toits potential use for plant growth.

2. Materials and methods

2.1. Raw materials

Clam wastes were collected from a local seafood manufacturerat Norfolk, Virginia, USA, and were stored in a freezer at �20 oCprior to use. The major components of the material were offaland shells, with a moisture content of 56%. The woodchips (bulkingagents) were made from hardwood and collected from a localwood processor (Chesapeake Hardwoods Inc., VA, USA). The mate-rials were shredded in a large hog with an average size of 5–20 mm, which is recommended for good aeration of compostingsystems (Biddlestone and Gray, 1985). The moisture content ofthe woodchips was 43%.

2.2. Laboratory-scale composting

A ‘‘self-heating”-type reactor that relies on microbial heat pro-duction to reach and maintain the temperature (Mason and Milke,2005) was used in the laboratory-scale composting study. Thereactors were 5-L rectangular foam containers with dimensionsof 20.5 cm � 15.5 cm � 17.0 cm (length �width � height). Thecontainers were perforated with 1–2 holes (0.6–0.8 cm diameter)on each insulation wall (7–8 cm thickness) to achieve natural ven-tilation. In addition, a silicon tube (0.6 cm diameter) was insertedinto the reactor so as to end in the ‘‘core” of the reactor. This tubeserved as a duct for passive aeration of the reactor.

Clam wastes and woodchips were mixed at ratios of 1:0.5, 1:1,1:1.5, 1:2, and 1:3 (w/w, wet weight). The moisture content of themixes was adjusted by sprinkling water onto the mixes. Threeidentical reactors were used for each ratio. During composting,the temperature and weight loss of the mixes were monitored.Several locations inside the reactor were monitored, and the reac-tor temperature was reported as the average of those readings.

2.3. Pilot-scale composting

Two identical 120-L drums (75 cm height with 40 cm innerdiameter) were used for pilot-scale composting. The reactors were

Table 1Characteristics of clam wastes, woodchips, and their mixes at different ratiosa

Parameters Raw materials Raw m

Clam wastes Woodchips 1:0.5

Moisture (%) 56.5 ± 1.2 43.2 ± 0.9 59.6 ±pH 6.21 ± 0.15 7.75 ± 0.05 6.13 ±Electrical conductivity (mS/cm�1) 26.5 ± 2.4 2.5 ± 0.3 14.2 ±Organic matter (g/kg dry matter) 800.0 ± 17.2 990.9 ± 2.5 875.4 ±Ash (g/kg dry matter) 200.0 ± 17.2 9.1 ± 2.5 124.6 ±Organic carbon (g/kg dry matter) 432.0 ± 9.5 535.9 ± 1.4 472.7 ±TKN (g/kg dry matter) 102.1 ± 3.5 18.1 ± 1.2 68.9 ±C/N 4.2 29.6 6.9Organic-N (g/kg dry matter) 98.6 ± 3.7 18.0 ± 1.1 66.7 ±NH4-N (g/kg dry matter) 3.5 ± 0.3 0.1 ± 0.01 2.1 ±Phosphorus (g/kg dry matter) 3.96 ± 0.11 0.13 ± 0.04 3.01 ±Potassium (g/kg dry matter) 0.48 ± 0.04 0.79 ± 0.04 0.60 ±

a Data are mean values ± SD of three replicates.

wrapped with glass-wool (8 cm thickness) for insulation. The top,bottom, and sides of each drum were perforated with a total of30 holes (1.5 cm diameter) for natural aeration. The reactors weremanually shaken 2–3 min every day to enhance the mixing andaeration. The initial ratio of clam waste to woodchips was set at1:1 (w/w, wet weight). During composting, the dynamic changeof moisture content was monitored and maintained at 55–65%.The temperature was measured by inserting a Traceable� long-stem thermometer (VWR, International, PA, USA) into six differentpositions of the reactor, and the average of those six readings wasreported as the reactor temperature.

2.4. Analysis

The moisture content was determined by drying the sample at105 oC for 24 h. Electrical conductivity (EC), pH, and ammoniumwere determined in the water extract according to the proceduresdescribed by Laos et al. (2002). In short, a wet sample (�20 g) wasdried at 60 oC and milled to pass through a 20#-mesh sieve. Thedried particles were then mixed with distilled water at a ratio of1:20 (w/w), and shaken at an incubator shaker for 2 h. The waterextract was filtered through waterman 1# filter paper; EC, pH,and ammonium of the filtrate were determined by an EC probe,pH probe, and ammonia-selective electrode, respectively.

Organic matter and ash content were determined by burningthe dried sample at 550 oC for 4 h (Clesceri et al., 1998). The organ-ic matter was converted to carbon content by a factor of 54% (Bar-rington et al., 2002; Castellanos and Pratt, 1981). Total Kjeldahlnitrogen (TKN) was determined by the semi-micro-Kjeldahl meth-od (Clesceri et al., 1998). Organic nitrogen (organic-N) was calcu-lated by subtracting ammonium-N from TKN. Various elementsincluding phosphorus, potassium, calcium, magnesium, sulfur, so-dium, iron, and trace amounts of zinc, manganese, copper, and bor-on of the samples were analyzed according to the US EPA 3050Bmethod (Anon, 1996).

3. Results and discussion

3.1. Characteristics of raw materials

Table 1 shows the characteristics of the clam waste, wood-chips, and their mixes at different ratios. Clam waste and wood-chips contained 56.5% and 43.2% of moisture, respectively. It hasbeen reported that the optimal moisture for efficient compostingis generally 40–60% (Liang et al., 2003; Singh et al., 2006); thus,water was sprinkled onto the mixes of clam waste/woodchips tokeep the moisture content at 59–67% (Table 1), so a high bioactiv-

aterial mix (clam waste: woodchips, w/w, wet weight)

1:1 1:1.5 1:2 1:3

1.4 67.2 ± 2.1 59.9 ± 0.9 65.2 ± 1.7 60.9 ± 1.40.14 6.29 ± 0.17 6.28 ± 0.12 6.37 ± 0.15 6.35 ± 0.091.5 9.1 ± 1.4 8.0 ± 0.9 5.5 ± 0.7 5.3 ± 0.714.3 908.1 ± 11.5 926.4 ± 18.7 938.0 ± 12.4 952.0 ± 8.814.3 91.9 ± 11.5 73.6 ± 18.7 62.0 ± 12.4 48.0 ± 8.87.9 490.4 ± 6.4 500.3 ± 10.4 506.5 ± 6.9 514.1 ± 4.95.4 54.5 ± 2.9 46.5 ± 2.3 41.4 ± 1.7 35.2 ± 1.4

9.0 10.8 12.2 14.65.2 52.9 ± 2.6 45.2 ± 2.2 40.3 ± 1.7 34.4 ± 1.30.1 1.6 ± 0.08 1.3 ± 0.06 1.1 ± 0.1 0.8 ± 0.060.17 2.09 ± 0.20 1.63 ± 0.06 1.54 ± 0.03 1.09 ± 0.020.06 0.66 ± 0.03 0.69 ± 0.07 0.71 ± 0.05 0.73 ± 0.07

182 Z. Hu et al. / Waste Management 29 (2009) 180–185

ity of the microorganisms and a good aeration capacity could bemaintained.

The clam waste had an acidic pH level. Due to the high salinityand substantial amount of shell debris contained in the material,the clam waste had a high electrical conductivity (EC) level, highash content, and low organic matter. The nitrogen content (TKN,organic-N, and ammonium-N) of the clam waste was also high.However, the woodchips had quite different characteristics, suchas slightly alkali pH, low EC and ash content, and low nitrogen con-tent (Table 1). When the two raw materials were mixed, the pH ofthe mixes was in the acidic range, and the C/N ratios ranged from

0 8 16 24 320

20

40

60

0 8 16 24 320

20

40

60

0 8 16 24 320

20

40

60

0 8 16 24 320

20

40

60

0 8 16 24 320

20

40

60

Time (d)

1:3

1:2

1:1.5

1:1

Tem

pera

ture

(ο C

)T

empe

ratu

re (

ο C)

Tem

pera

ture

(ο C

)T

empe

ratu

re (

ο C)

Tem

pera

ture

(ο C

)

1:0.5

Fig. 1. Temperature profiles of the laboratory-scale composting test with differentinitial ratios of clam waste to woodchips (data are mean values of three replicates;error bars show the standard deviations of the mean values).

6.9:1 to 14.6:1 (Table 1). This C/N ratio was much lower than thatof most waste streams such as pig litter, cattle manure, filter cake,and bagasse (Meunchang et al., 2005; Raviv, 2005; Tiquia and Tam,2000). Table 1 also shows that the total phosphorus (P) of clamwaste was much higher than that of woodchips, while potassium(K) content was lower than for woodchips.

3.2. Laboratory-scale composting

3.2.1. Temperature profile and weight loss during compostingTemperature is an important parameter to monitor composting

efficiency, because it affects not only the biological reaction ratesand the population dynamics of microbes, but also the physico-chemical characteristics of composts (Antizar-Ladislao et al.,2005; Namkoong et al., 2002). Fig. 1 shows the temperature changesof clam waste/woodchips mixes at different ratios. All mixes clearlyshow the four compost phases: mesophilic, thermophilic, cooling,and maturation phases. The high ratio of clam waste to woodchipsresulted in a higher temperature and longer thermophilic phaseand, thus, an improved composting performance (Fig. 1). The reasonwas that clam waste contained high levels of easily digestible com-ponents such as proteins; when mixed with woodchips, the mixeswith a high portion of clam waste generated more heat. Similarresults have been reported in fish waste composting, in which a highratio of fish waste to sawdust (bulking agent) attained a higher tem-perature (Liao et al., 1995; Laos et al., 2002).

Fig. 2 shows the weight losses of the clam waste/woodchipsmixes during composting. The whole process can be divided intothree phases: rapid weight loss phase (days 0–6), slow weight lossphase (days 6–26), and stable weight phase (days 26–30). Com-pared with the temperature profile, the rapid weight loss phasewas found to fall into the mesophilic and thermophilic stages,the slow weight loss happened in the cooling stage, and the stableweight phase indicates that the compost reached maturity (Figs. 1and 2). For the entire composting period, the total weight loss wassignificantly affected by the ratio of clam waste to woodchips.There were more than 30% weight losses for the ratios of 1:0.5,1:1 and 1:1.5; while the weight loss for the ratio of 1:2 was about21%, and the ratio of 1:3 was only 13%.

The high temperature observed in Fig. 1 is an index of highmicrobial activity, which induces the rapid decomposition of theorganic matter at the earlier stages of composting. This result is

0 8 16 24 320

10

20

30

40

50

Wei

ght l

oss

(%)

Time (d)

1:0.5 1:1 1:1.5 1:2 1:3

Fig. 2. Weight losses (wet weight) during laboratory-scale composting test withdifferent initial ratios of clam waste to woodchips (data are mean values of threereplicates; errors bars show the standard deviations of the mean values).

Table 3Weight losses (% of initial dry weight) of organic matter and major nutrients in thelaboratory-scale composting testa

Initial ratio ofclam waste towoodchips

Organicmatter(%)

Carbonloss (%)

Nitrogenloss (%)

Phosphorusloss (%)

Potassiumloss (%)

1:0.5 30.7 ± 2.4 30.2 ± 2.5 46.2 ± 1.2 6.4 ± 0.07 10.9 ± 0.061:1 22.5 ± 1.4 22.3 ± 1.5 40.7 ± 1.7 7.1 ± 0.1 15.6 ± 0.21:1.5 21.3 ± 1.3 20.8 ± 1.5 35.4 ± 2.7 8.3 ± 0.2 15.3 ± 2.61:2 8.9 ± 1.2 8.8 ± 1.4 31.5 ± 1.2 4.2 ± 0.2 15.1 ± 3.41:3 12.4 ± 0.9 12.4 ± 1.4 26.6 ± 1.4 6.9 ± 1.6 14.7 ± 1.8

a Data are mean values ± SD of three replicates.

Z. Hu et al. / Waste Management 29 (2009) 180–185 183

probably due to the large amount of readily digestible components(e.g., proteins) contained in the clam waste, which were immedi-ately available for microbes to utilize. The high temperature alsoinduces the weight loss through the evaporation of ammoniawhich derived form protein decomposition. Another reason forweight loss is due to the leachate loss from the reactor. Althoughthe composition of this leachate was not characterized in thiswork, it is thought to contain various mineralized ions, which iseventually lost from the composting materials (see Section 3.2.3).When readily digestible compounds were exhausted, the recalci-trant materials, such as cellulose and hemicellulose contained inthe woodchips, were slowly degraded (Komilis and Ham, 2003).A different weight loss pattern has been reported in lignocellu-loses-rich waste materials. For example, in composting olive-millbyproducts, a slow degradation of the lignocelluloses into sugarshappened before other microbes could further utilize the sugarsat a high rate; therefore, a slight and slow weight loss happenedbefore a rapid weight loss (Alburquerque et al., 2006).

3.2.2. Characteristics of the composts3.2.2.1. Moisture content, pH, and electrical conductivity. Moisturecontent has significant effects on enzyme activities and microbialrespiration of the composting process (Margesin et al., 2006). Ingeneral, a 50% moisture was the minimum requirement for main-taining high microbial activity (Liang et al., 2003). In this work, itwas found that the moisture contents of the final composts (50–60%) were slightly lower than those of the initial materials (Tables1 and 2). The compost with a high ratio of clam waste to wood-chips had a greater moisture loss (Table 2).

The pH of the composts was in the neutral range (Table 2). Com-pared with the initial pH (Table 1), the pH of the final compostsshows a little increase, probably due to the decomposition of pro-teins to ammonium, which was also reported by other researchers(Liao et al., 1995; Raviv, 2005). Similar to pH changes, EC was in-creased after 30 days composting (Tables 1 and 2). This result isattributed to the release of several ions during the mineralizationof organic matter.

3.2.2.2. Organic matter and ash. The clam waste contained a fairamount of shell residues; the ash contents of the raw materials,especially the mixes with a high ratio of clam waste to woodchips,were at high levels. The high ash contents resulted in relatively lowproportions of organic matter (Table 1). During composting, the or-ganic matter was degraded into volatile compounds and was lostfrom the solid compost. As a result, the final compost had an evenlower organic-matter level (Table 2).

3.2.2.3. Carbon, nitrogen, phosphorus, and potassium. Table 2 showsthat the organic carbon of the compost increased with decreasing

Table 2Characteristics of composts with different initial ratios of clam waste to woodchips in the

Initial ratio of clam waste to woodchips

1:0.5 1:1

Moisture (%) 50.1 ± 2.4 50.8 ± 0.1pH 7.03 ± 0.01 7.08 ± 0.10Electrical conductivity (mS cm�1) 16.9 ± 0.1 12.4 ± 0.2Organic matter (g/kg dry matter) 812.1 ± 1.1 871.2 ± 9.4Ash (g/kg dry matter) 187.9 ± 1.1 128.8 ± 9.4Organic carbon (g/kg dry matter1) 450.7 ± 5.5 483.5 ± 3.8TKN (g/kg dry matter) 49.6 ± 4.0 40.0 ± 1.4C/N 9:1 12:1Organic-N (g/kg dry matter) 32.9 ± 1.8 29.4 ± 1.1NH4-N (g/kg dry matter) 16.7 ± 2.2 10.6 ± 0.2Phosphorus (g/kg dry matter) 3.76 ± 0.26 2.40 ± 0.04Potassium (g/kg dry matter) 0.76 ± 0.06 0.69 ± 0.008

a Data are mean values ± SD of three replicates.

ratio of clam waste to woodchips, TKN and phosphorus decreased,while potassium was almost unchanged. The C/N ratios of the com-post were in the range from 9:1 to 18:1. Compared to the initialraw materials (Table 1), the final compost had decreased organicnitrogen (organic-N) and increased ammonium-N (Table 2), dueto the decomposition of proteins in the clam waste. Similar resultswere observed in the composting of fish waste, which also con-tained a high level of proteins (Laos et al., 2002). Nitrate was notdetected in the compost, probably due to an inadequate O2 leveland a lack of nitrification bacteria in the composting materials.

3.2.3. Losses of organic matter and major nutrientsThe net losses of organic matter and major nutrients (carbon,

nitrogen, phosphorus, potassium) during composting are shownin Table 3. The loss of organic matter was at a high level whenthe composting mixes contained a high portion of clam waste. Inthe composting process, carbon is lost in the form of CO2 and nitro-gen in the form of ammonia (Barrington et al., 2002). Table 3 showsthat there were about 8–30% of carbon losses and 26–47% of nitro-gen losses; mixes with a high portion of woodchips had less C andN losses, perhaps because of the high capabilities of cation/ammo-nia absorption and fixing by woodchips (Barrington et al., 2002).The phosphorus and potassium losses were in the range of 4.2–8.3% and 5.9–15.6%, respectively. During composting, these twoelements were mineralized, but not in a volatile form (Tiquiaet al., 2002); therefore, their losses may be attributed to the disso-lution of the two elements in the leachate, which were eventuallyremoved from the composting materials.

The above results demonstrate the feasibility of compostingclam waste with woodchips at laboratory-scale. Based on these re-sults, a pilot-scale composting test was further conducted to inves-tigate the scale-up of the process. The ratio of clam waste towoodchips in the pilot-scale test was set at 1:1 (w/w, wet weight)because this ratio had previously shown the best temperature per-formance (Fig. 1).

laboratory-scale composting testa

(clam waste: woodchips, w/w, wet weight)

1:1.5 1:2 1:3

54.9 ± 0.9 56.8 ± 0.1 59.5 ± 0.17.14 ± 0.09 7.37 ± 0.07 7.39 ± 0.06

9.3 ± 0.5 9.8 ± 0.1 8.2 ± 0.2899.2 ± 2.7 909.8 ± 16.7 938.9 ± 3.4100.8 ± 2.7 90.2 ± 16.7 61.1 ± 3.4485.6 ± 4.4 504.9 ± 4.6 521.1 ± 3.8

32.6 ± 2.4 31.0 ± 3.8 29.1 ± 0.515:1 16:1 18:124.4 ± 2.5 21.6 ± 4.2 23.5 ± 0.6

8.3 ± 0.2 9.4 ± 0.4 5.6 ± 0.11.60 ± 0.09 1.62 ± 0.016 1.14 ± 0.0080.68 ± 0.00 0.66 ± 0.00 0.70 ± 0.01

0 10 20 30 40 50 600

20

40

60

80

Tem

pera

ture

(ο C

)

Time (d)

Composting temperature Ambient temperature

Fig. 3. Temperature profile of the pilot-scale composting test (data are mean valuesof two replicates; error bars show the deviations from the mean values).

0

5

10

15

0

20

40

60

0 15 30 45 60

0 15 30 45 60

0 15 30 45 60

0

4

8

12

16

C/N

Time (d)

CC

once

ntra

tion

(g/k

g dr

y m

atte

r)

TKN

Organic-N

NH4-N

B

Ele

ctri

cal c

ondu

ctiv

ity

(mS

cm-1

)

A

Fig. 4. Dynamic changes in EC value (A), nitrogen content (B) and C/N ratio (C) ofclam waste/woodchips mix during pilot-scale composting test (data are mean va-lues of two replicates; error bars show the deviations from the mean values).

184 Z. Hu et al. / Waste Management 29 (2009) 180–185

3.3. Pilot-scale composting

3.3.1. Temperature profileFig. 3 shows the temperature profile of pilot-scale composting.

During the composting period, the ambient temperature wasmaintained at around 20 oC. The temperature of the compost in-creased sharply to 68 oC within the first 3 days. From day 3 today 9, the temperature stayed above 55 oC, although it graduallydecreased from its highest level of 68 oC. The composting mixcooled down to room temperature after 33 days, and kept steadyfor another 15 days for maturation. Compared with the labora-tory-scale test (1:1 ratio, Fig. 1), the pilot-scale composting testdemonstrated a higher temperature and a longer thermophilicstage (Fig. 3).

It has been reported that a composting temperature above 55 oCcan kill pathogens and sanitize the compost (Cekmecelioglu et al.,2005; Gea et al., 2005). Indeed, the US EPA requires that the com-posting temperature be kept above 55 oC for at least 5 days for san-itary purposes (Liang et al., 2003). Thus, the temperature profileobtained from the pilot-scale composting test (Fig. 3) met the san-itary requirements of the US EPA (Anon, 1993).

3.3.2. Dynamic changes of electrical conductivity, nitrogen, and C/Nratio during composting

Due to the specific characteristics of clam waste, the initial clamwaste/woodchips mix had a high salinity, high nitrogen content,and low C/N ratio. Therefore, the dynamic changes of these threeparameters were monitored in order to give insight into the com-posting performance.

As shown in Fig. 4A, the initial EC value was 8.7 ± 0.8 mS cm�1;during the first 40 days, the EC value gradually increased from8.7 ± 0.8 to 13.9 ± 0.9 mS cm�1, indicating mineralization of organ-ic matter into soluble salts. There was no significant EC changefrom day 40 to day 53 (Fig. 4A). Fig. 4B shows that in the first 8days, the organic nitrogen level decreased sharply, while ammo-nium-N increased. These trends indicate a rapid protein decompo-sition. After day 8, there was no significant decrease of organicnitrogen, while ammonium-N had a slight decrease due to contin-uous evaporation from the composting materials (Fig. 4B). Duringthe composting process, the C/N ratio of the composting mix in-creased (Fig. 4C). Such a trend of the C/N ratio was quite differentfrom that of composting of other types of waste materials such asanimal manure and agricultural residues (Adler and Sikora, 2005;Meunchang et al., 2005; Raviv, 2005), in which the C/N ratio in-creased with composting time because more carbon than nitrogenwas lost. The trend of increased C/N with composting time ob-

served in this work was caused by the high nitrogen content con-tained in the clam waste.

3.3.3. Characteristics of matured compostCharacteristics of the matured compost obtained from the pilot-

scale composting test are shown in Table 4. The matured composthad a slightly acidic pH. Compared with other types of compostmaterials (Caceres et al., 2006), the compost derived from the clamwaste/woodchips mix had a higher EC value and higher ash content.

Although the initial C/N of the clam waste/woodchips mix wasmuch lower than that of most other types of waste materials (Adlerand Sikora, 2005; Meunchang et al., 2005; Raviv, 2005), the C/N ra-tio of the matured compost was 14.6:1, which is similar to that ofother matured composts. A relatively high content of ammoniumwas observed (Table 4), but nitrate was not detected in the finalproduct. Similar results have been reported in composts from ol-ive-mill byproducts and fish (Alburquerque et al., 2006; Laoset al., 2002; Liao et al., 1995). Because a high ammonia contentin the final compost may cause significant ammonia volatilizationand N losses (Raviv, 2005), a nitrifying bacteria may need to beadded in the future for composting clam waste material.

In addition to carbon and nitrogen, the matured compost alsocontained a variety of nutrients such as phosphorus, potassium, cal-cium, magnesium, sulfur, sodium, iron, and trace amounts of zinc,manganese, copper, and boron (Table 4). It has been reported thatmore than 80% of potassium and sodium are likely immediately

Table 4Characteristics of compost from the pilot-scale composting testa

Parameters

pH 6.54 ± 0.11Electrical conductivity (mS cm�1) 14.5 ± 1.3Organic matter (g/kg dry matter) 833.0 ± 14.1Ash (g/kg dry matter) 167.0 ± 14.1Organic carbon (g/kg dry matter) 449.8 ± 7.8TKN (g/kg dry matter) 30.8 ± 2.2C/N 14.6Organic-N (g/kg dry matter) 27.5 ± 1.4NH4-N (g/kg dry matter) 6.5 ± 0.3Calcium (g/kg dry matter) 12.9 ± 1.3Magnesium (g/kg dry matter) 8.0 ± 0.5Phosphorus (g/kg dry matter) 2.15 ± 0.10Potassium (g/kg dry matter) 0.79 ± 0.06Sulfur (g/kg dry matter) 3.00 ± 0.21Sodium (g/kg dry matter) 1.15 ± 0.12Iron (g/kg dry matter) 1.2 ± 0.1Zinc (mg/kg dry matter) 24.9 ± 2.1Manganese (mg/kg dry matter) 80.4 ± 5.5Copper (mg/kg dry matter) 5.7 ± 0.2Boron (mg/kg dry matter) 5.7 ± 0.3

a Data are mean values ± deviation of two replicates.

Z. Hu et al. / Waste Management 29 (2009) 180–185 185

available for plant growth. The availability of phosphorus, calcium,and magnesium ranges from 25% to 75%; more of these elements areavailable under acidic conditions (Woods End Research Laboratory,2005). The above chemical compositions suggest that clam wastecompost could potentially be used for plant growth. However, itshould be noted the raw clam waste has a high EC value; the ma-tured compost had an even higher EC value. Such ‘‘salty” character-istics should be considered when applying the compost for plantgrowth because a large amount of soluble minerals may inhibit bio-logical activity. Therefore, the clam waste/woodchips compost ob-tained here can only be used at low applications. An alternativemethod such as anaerobic digestion might overcome this high ECproblem. In an anaerobic digestion process, a large amount of waterwould be mixed with clam waste in order to keep the material in aflowable slurry state. This practice reduces the EC value and, thus,makes the digested materials more suitable for plant growth.

4. Conclusion

In summary, the above results indicate that composting withwoodchips can be used as a method for treating clam waste mate-rial. The ratio of clam waste to woodchips had a significant influ-ence on composting performance, with a ratio of 1:1 of clamwaste to woodchips being the optimal ratio based on the temper-ature profiles in laboratory scale compositing. The temperature ob-tained in the pilot-scale composting test met the US EPA sanitaryrequirement for pathogen-killing. Because of the special character-istics of the raw clam waste, the matured compost had high levelsof salinity, ash content, and nitrogen content. The final compostproduct had a C/N ratio of 14.6:1, and an organic matter of833.0 ± 14.1 g/kg dry matter, with a considerable amount of cal-cium, magnesium, phosphorus, potassium, and other micronutri-ents, which are a good source of plant nutrients.

Acknowledgments

The authors acknowledge financial support from the VirginiaCommercial Fishery and Shellfish Technologies (C-FAST) program,and Virginia Tech New Faculty Start-up Fund (to ZW).

References

Adler, P.R., Sikora, L.J., 2005. Mesophilic composting of Arctic char manure. CompostScience & Utilization 13, 34–42.

Alburquerque, J.A., Gonzalvez, J., Garcia, D., Cegarra, J., 2006. Composting of a solidolive-mill by-product (‘‘alperujo”) and the potential of the resulting compost forcultivating pepper under commercial conditions. Waste Management 26, 620–626.

Anon., 1993. Standard for the use or disposal of sewage sludge. US EnvironmentalProtection Agency Federal Register: 58. pp. 9248–9415.

Anon., 1996. Inductively Coupled Plasma-atomic Emission Spectrometry. USEnvironmental Protection Agency EPA 6010B.

Antizar-Ladislao, B., Lopez-Real, J., Beck, A.J., 2005. Laboratory studies of theremediation of polycyclic aromatic hydrocarbon contaminated soil by in-vesselcomposting. Waste Management 25, 281–289.

Barrington, S., Choiniere, D., Trigui, M., Knight, W., 2002. Effect of carbon source oncompost nitrogen and carbon losses. Bioresource Technology 83, 189–194.

Biddlestone, A.J., Gray, K.R., 1985. Composting. In: Moo-Young, M. (Ed.),Comprehensive Biotechnology: The Principles, Applications, and Regulationsof Biotechnology in Industry, Agriculture, and Medicine. Pergamon Press,Oxford. pp. 1059–1070.

Caceres, R., Flotats, X., Marfa, O., 2006. Changes in the chemical andphysicochemical properties of the solid fraction of cattle slurry duringcomposting using different aeration strategies. Waste Management 26, 1081–1091.

Castellanos, J.Z., Pratt, P.F., 1981. Mineralization of manure nitrogen-correlationwith laboratory indexes. Soil Science Society of America Journal 45, 354–357.

Cekmecelioglu, D., Demirci, A., Graves, R.E., Davitt, N.H., 2005. Optimization ofwindrow food waste composting to inactivate pathogenic microorganisms.Transactions of the ASAE 48, 2023–2032.

Clesceri, L.S., Greenberg, A.E., Eaton, A.D., 1998. Standard Methods for theExamination of Water and Wastewater, 20th ed. American Public HealthAssociation, Washington, DC, USA.

Gea, T., Artola, A., Sort, X., Sanchez, A., 2005. Composting of residuals produced inthe Catalan wine industry. Compost Science & Utilization 13, 168–174.

Hue, N.V., Sobieszczyk, B.A., 1999. Nutritional values of some biowastes as soilamendments. Compost Science & Utilization 7, 34–41.

Komilis, D.P., Ham, R.K., 2003. The effect of lignin and sugars to the aerobicdecomposition of solid wastes. Waste Management 23, 419–423.

Laos, F., Mazzarino, M.J., Walter, I., Roselli, L., Satti, P., Moyano, S., 2002. Compostingof fish offal and biosolids in northwestern Patagonia. Bioresource Technology81, 179–186.

Liang, C., Das, K.C., McClendon, R.W., 2003. The influence of temperature andmoisture contents regimes on the aerobic microbial activity of a biosolidscomposting blend. Bioresource Technology 86, 131–137.

Liao, P.H., May, A.C., Chieng, S.T., 1995. Monitoring process efficiency of a full-scalein-vessel system for composting fisheries wastes. Bioresource Technology 54,159–163.

Margesin, R., Cimadom, J., Schinner, F., 2006. Biological activity during compostingof sewage sludge at low temperatures. International Biodeterioration &Biodegradation 57, 88–92.

Mason, I.G., Milke, M.W., 2005. Physical modelling of the composting environment:a review part 1: reactor systems. Waste Management 25, 481–500.

Meunchang, S., Panichsakpatana, S., Weaver, R.W., 2005. Co-composting of filtercake and bagasse; by-products from a sugar mill. Bioresource Technology 96,437–442.

Namkoong, W., Hwang, E.Y., Park, J.S., Choi, J.Y., 2002. Bioremediation of diesel-contaminated soil with composting. Environmental Pollution 119, 23–31.

Nicholls, D., Richard, T., Micales, J.A., 2002. Wood and fish residuals composting inAlaska. Biocycle 43, 32–34.

Nicolas, J., Craffe, F., Romain, A.C., 2006. Estimation of odor emission rate fromlandfill areas using the sniffing team method. Waste Management 26, 1259–1269.

Papadopoulou, M.P., Karatzas, G.P., Bougioukou, G.G., 2007. Numerical modelling ofthe environmental impact of landfill leachate leakage on groundwater quality—a field application. Environmental Modeling & Assessment 12, 43–54.

Raviv, M., 2005. Production of high-quality composts for horticultural purposes: amini-review. Horttechnology 15, 52–57.

Ros, M., Garcia, C., Hernandez, T., 2006. A full-scale study of treatment of pig slurryby composting: kinetic changes in chemical and microbial properties. WasteManagement 26, 1108–1118.

Singh, A., Billingsley, K., Ward, O., 2006. Composting: a potentially safe process fordisposal of genetically modified organisms. Critical Reviews in Biotechnology26, 1–16.

Tiquia, S.M., Richard, T.L., Honeyman, M.S., 2002. Carbon, nutrient, and mass lossduring composting. Nutrient Cycling in Agroecosystems 62, 15–24.

Tiquia, S.M., Tam, N.F.Y., 2000. Co-composting of spent pig litter and sludge withforced-aeration. Bioresource Technology 72, 1–7.

Woods End Research Laboratory, 2005. Interpreting Waste & Composting Tests.<http://www.woodsend.org/pdf-files/compost.pdf> (accessed 2.01.2007).

Zubillaga, M.S., Lavado, R.S., 2006. Phytotoxicity of biosolids compost at differentdegrees of maturity compared to biosolids and animal manures. CompostScience & Utilization 14, 267–270.