Respiration profiles in monitoring the composting of by-products from the olive oil agro-industry Ioanna Mari a,b , Constantinos Ehaliotis a,b, * , Maria Kotsou c , Costas Balis c , Dimitrios Georgakakis a a Division of Soils and Agricultural Chemistry, Department of Natural Resources Reclamation and Agricultural Engineering, Agricultural University of Athens, Iera Odos 75, Athens 118 55, Greece b National Agricultural Research Foundation, Institute of Kalamata, 85 Lakonikis St., Kalamata 24100, Greece c Laboratory of Microbiology, Harokopio University of Athens, 70 Eleutheriou Venizelou St., Athens 176 71, Greece Received 6 November 2001; received in revised form 12 May 2002; accepted 3 September 2002 Abstract Thecompostingofolivepresscake(OPC)repeatedlymixedeitherwitholivemillwastewater(OPC+OMW)orwithtapwater (OPC+W)wasstudiedusingthethermogradientrespirometer,anapparatusthatdeterminestherespirationratesfromasubstrate overawiderangeofdifferenttemperatures(respiratoryprofile).Thecompostingprocessestookplaceoveraperiodoffivemonths duringwhichninemoisteningsoftheOPCwereperformedwiththerespectiveliquids.Thecompostingresultedindetoxificationof thematerialsusedinbothtreatments,asindicatedbyseedgerminationtests.However,therepeatedapplicationsofOMWresulted in recurring thermophilic phases (following each application) and in greater pH and conductivity increases in the final product, as compared to water applications. Respiration measurements performed at 35 °C were good indicators of the mean metabolic po- tential in the compost piles (the mean respiration derived from the whole respiration profile over a wide range of environmental temperatures).However,respirationmeasurementsathighertemperatures(48.5 °C)werebetterindicatorsoftherespirationactivity occurringinsitu.Followingtheinitialthermophilicphase,therespirationpotentialofthecompostsathightemperatures(42–63 °C) increased drastically compared to their respiration potential at lower temperatures (17–42 °C) indicating the establishment of a thermophilic microflora. Subsequently, only the periodic new substrate-C applications in the form of OMW resulted in increased ratios of low temperature-to-high temperature respiration potential. These ratios decreased again following the respective thermo- philic phase that each new OMW application had induced. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Compost; Olive mill wastewater; Olive press cake; Microbial activity; Respirometry; Compost respiration; Thermophilic 1. Introduction During the last decades research has focused on the qualitative evaluation of the composting process. Sev- eral indicator variables have been proposed for moni- toring the process and evaluating the stability of the final product (Morel et al., 1985; Mathur et al., 1993). Theseincludebothchemicalandbiologicalindicessuch as C/N ratios, cation exchange capacity (CEC), humus content and quality, phytotoxicity, respiration, dehy- drogenase activity, and arginine ammonification (For- ster et al., 1993; Harada and Inoko, 1980; Hirai et al., 1985;Saviozzietal.,1992;Inbaretal.,1990;Balisetal., 1995; Adani et al., 1995; Hue and Liu, 1995). Biological and biochemical indices are particularly useful since they relate composting processes to meta- bolicactivity.TotalATP,enzymeactivityandmicrobial biomass measurements have been employed, but the coregroupofmethodsisbasedonrespirometry.Oxygen is consumed and carbon dioxide is evolved as a conse- quenceofmicrobialmetabolism.Bothprocessesdecline at late composting stages (Ribalda et al., 1987). A wide range of devices has been developed for the measure- ment of respiration from solid matrices. Perhaps the most common is the ‘‘Warburg’’ apparatus, measur- ing changes of pressure in closed systems (manometric respirometer). Another approach includes electrolytic respirometric units in which the manometric changes Bioresource Technology 87 (2003) 331–336 * Corresponding author. Tel.: +30-1-5294098; fax: +30-1-5294092. E-mail address: ehaliotis@aua.gr (C. Ehaliotis). 0960-8524/03/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII:S0960-8524(02)00238-9