Composting

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Composting is a controlled microbial bio-oxidative process that involves a heterogeneous organic substrate in the solid state, which evolves through a thermophilic stage and the temporary release of phytotoxins, leading to the production of carbon dioxide, water, mineral salts, and stabilized waste containing humic-like substances. Composting of olive-mill wastes has been examined as a potential bioremediation treatment of these wastes (Vlyssides A.G. et al., 1999; HR20010028, 2002). By using this method, it is possible to transform either fresh OMWW or sludge from pond-stored OMWW mixed with appropriate plant waste materials (carriers) into organic fertilizers (composts) with no phytotoxicity to improve soil fertility and plant production (Paredes C., 1998, Paredes C. et al., 1996a). Composting can be put into effect by means of a mixture of solids with agricultural waste, essentially as cereal straw, sawdust, or the remaining solid waste from the olive-mill.

OMWW contains on average about 6% of organic matter and 0.4% of mineral salts suspended or dissolved in an aqueous medium. Therefore, their bioremediation through composting must be achieved by adding other materials having a high absorbing capacity, such as agricultural lignincellulose residues. The latter are very poor in nitrogen, usually present in an organically bound form, so a rapidly available nitrogen source is necessary to assure the C/N ratio required for microbial development. A composting process developed by the EU project: ETWA-CT92-0006 used OMWW-wheat straw mixtures. Both physico-chemical aspects of the process and the quality of the end-product were analyzed in a number of following papers (Galli E. et al., 1994, 1997; Tomati U. et al., 1995). Tomati U. et al. (1995) used chopped wheat straw and urea to compost OMWW containing ~7% solids in a forced-aeration static pile. The urea was added to ensure a C/N ratio of ~35. A rapid increase of microorganisms and bio-reactions occurred at the beginning of the process, which led to an increase of the temperature and pH and a decrease of total organic carbon. Degree of humification, the humification rate, and the humification index, respectively, reached the values of 78, 37.8, and 0.28% after two months. A lignin degradation of ~70% was assayed at the end of the thermophilic phase.

No phytotoxicity was recorded on the end product, the chemical and physical properties of which suggest its possible use as fertilizer. Galli E. et al. (1997) studied also the composting of an OMWW-wheat straw mixture. Two aspects of carbon compound metabolism — lignin degradation and bioconversion of phenols — were particularly investigated. Lignin is one of the main components of the mixture and the most resistant fraction in composting materials. It is closely associated with cellulose fibers and hampers the degradation of polysaccharides. Moreover, the aromatic units released during its degradation are essential building blocks for the biosynthesis of the humic substances. Phenols are assumed to be responsible for phytotoxicity and their bioconversion is very important for humic acid biosynthesis. Oxygen consumption, microbial growth and urease activity were greatly enhanced during the thermophilic phase, reaching their maximum in about three weeks. Casein-hydrolyzing protease showed a high initial activity, which sharply decreased after two weeks. The high initial value of protease and the rapid increase of urease activity indicate that nitrogen sources are promptly utilized by the growing microflora. The development of the thermophilic microorganisms, particularly fungi, allows the degradation of lignin. The degradability of the OMWW-wheat straw mixture is made evident by the great oxygen consumption. At the end of the thermophilic phase both phenols and lignin were reduced by about 70%. Composting enhanced diazotrophic microflora as indicated by nitrogenase activity which increased at the end of the thermophilic phase.

The evaporative capacity of an intensive composting process was employed to treat OMWW. A mixture of extracted olive press cake and olive tree leaves was used as the solid substrate for composting. OMWW was added to the composting mass to replenish the water loss during processing in a pilot-scale open static container reactor. The salinity content of the compost was the factor restricting the treatment of OMWW by the process. The rate of OMWW treatment achieved in this study was 2.1 l/kg starting solid substrate (dry weight). The cumulative moisture and volatile solids content reduction during the temperature-induced aeration period of the process was 19 and 45%, respectively (Papadimitriou E.K. et al., 1997).

Della Monica M. et al. (1980) processed OMWW in a tank filled with soil. The effect of the treatment is an enrichment of the soil with readily assimilable nutrient sub-stances to the extent that the soil pollutant mixture becomes soil-compost. Neither sludge nor solid residual products were formed in the process, since they undergo degradation too. The treatment of OMWW and sludge is completed on parcels of land underlain with a waterproof base. The waterproof floor prevents filtration of polluting substances in the treated wastewater from percolating into the underlying soil.

IT1244520 (1994) describes a process and plant for the treatment of OMWW, in which OMWW is poured onto a layer of agricultural earth contained in a tank, the pollutant substances contained in OMWW undergo a degradation process by means of the said agricultural earth, and finally this earth, transformed into soil-compost with fertilizing characteristics, is subjected to washing out with water in order to remove and recover the soluble salts.

Negro Alvarez M.J. and Solano M.L. (1996) evaluated the quality of different products obtained through the composting of the solid residue that results from the flocculation of OMWW. To facilitate composting, the residue of flocculation was mixed with different lignocellulosic residues (straw, vine shoots, olive branches, and olive stone). The composting was carried out in a climatic chamber in PVC containers having a capacity of 5l. Samples were periodically taken which were characterized and analyzed. Except for the mixture of the residue of flocculation with olive stone, in the rest of the mixtures assayed, an important degradation of organic matter as well as cellulose was observed. In addition, a decline of phytotoxicity, which the initial product presented, was observed. The results obtained show that the composting of this residue, when mixed with others of lignocellulosic character, is an effective manner of resolving the problem, while generating quality products from the point of view of its agricultural utilization.

Co-composting of olive cake and OMWW has been investigated as a potential bioremediation treatment for these wastes. Experimental results from a demonstration plant using olive cake as a bulking material and OMWW in a continuous feed have been reported by Vlyssides A.G. et al. (1996) — see Fig. 8.6. Composting temperature was controlled at 45-65°C and OMWW addition was fed in as necessary to maintain moisture content of 45-60% and to replenish the carbon

Fig. 8.6. Flow diagram of the demonstration plant (Vlyssides A.G. et al., 1996). A, OMWW feed; B, feed storage tank; C, co-composting bioreactor; D, urea feed system; E, agitator; F, air feeding fans; G, roof to prevent access of rainwater; H, mono-pump for OMWW dosing; I, proportional pump feeding urea solution; K, computer for controlling and data collection; L, traveling bridge for the agitator; M, motors; TC, temperature controller.

Fig. 8.6. Flow diagram of the demonstration plant (Vlyssides A.G. et al., 1996). A, OMWW feed; B, feed storage tank; C, co-composting bioreactor; D, urea feed system; E, agitator; F, air feeding fans; G, roof to prevent access of rainwater; H, mono-pump for OMWW dosing; I, proportional pump feeding urea solution; K, computer for controlling and data collection; L, traveling bridge for the agitator; M, motors; TC, temperature controller.

substrate level. During 23 days of operation at thermophilic temperature, a total of 263 m3 of effluent was treated and an estimated total of 90,00,000 kcal of total bioenergy was generated. The 23-day thermophilic period was followed by a 3-month mesophilic stabilization period. The resulting composted product is suitable as a high-quality soil conditioner.

Filippi C. et al. (2002) evaluated also the possibility of co-composting olive cake and OMWW. The pH, E.C., total C and N, humic substances, phenolics, volatile acids, lipids, P and K values plus yeast, fungi, heterotrophic, cellulolytic and nitrifying bacteria, and phytotoxicological parameters were monitored during a 120-day stabilization process. Performance of the composting system adopted, together with physico-chemical characteristics of starting material and final product, are reported. Co-composting was found to induce a high level of organic matter change, with decrease of organic carbon, total nitrogen, and C/N ratio, as well as of the easily biodegradable lipids. Good metabolic activity of the microbiological population, with the starting material was also observed. The results obtained suggested that co-composting might be an adequate low-cost strategy for the recycling of olive-mill by-products.

Paredes C. et al. (1996b, 2000, 2002) studied the influence of a bulking agent on the degradation of OMWW during its co-composting with agricultural wastes. Two different piles prepared with OMWW sludge and either maize straw or cotton waste as bulking agents were composted by the Rutgers static pile system in a pilot plant, with the aim of ascertaining the most suitable conditions for degrading the OMWW sludges through composting. The use of maize straw, instead of cotton waste, as a bulking agent led to the following effects on the composting process of the OMWW sludges: (1) a lower mineralization of the organic matter at the end of the active phase of the process; (2) lower total-nitrogen losses by NH3-volatilization; (3) a higher biological nitrogen fixation, and (4) production of a stabilized organic matter with less humic characteristics. The phytotoxic effects in the pile with maize straw lasted for a longer time, probably due to its slower rate of organic matter mineralization. However, no phytotoxic effects were observed in both mature composts.

The sea grass Posidonia oceanica25 has been used for the production of organic compost or compost for agriculture, with co-composting of organic waste of agricultural, animal, or industrial units — see Fig. 8.7. The procedure applied comprises the collection and transfer of the sea grass to the treatment unit and then mixing with various organic wastes such as OMWW (Posidonia oceanica 67%, goat manure 20%, and OMWW 13%) and olive cake (Posidonia oceanica 67%, olive cake 23%, grape pomace 6%, olive leaves 1.5%, and sheep manure 2.5%), so that the C/N ratio is approximately 30:1 in the product of mixing. These ratios favor the growth of microorganisms, which control the biological composting process and help sea grass,

25Posidonia oceanica is not an alga, it is in fact a marine plant (phanerogam) which produces flowers. Posidonia meadows can only be found in the Mediterranean sea. Its role is incredibly important for the local ecosystems since many other species find their nutrients and housing in Posidonia meadows.

Fig. 8.7. Posidonia oceanica.

which is slow to break down naturally, to decompose and release its nutrients. The whole procedure lasts 9-12 months, in two phases. The end product is used as a means of plant growth with fertilizing properties, as a means to improve and enrich soil, as a means against soil erosion and exhaustion, as a product for land reclamation, as a product suitable for reforestation, as a crop-protective agent, and as organic material suitable to be mixed with metal compounds and minerals from industrial units (GR1003611, 2001).

In general, composting seems to be a feasible method to eliminate the toxicity of olive-mill wastes and to turn them into a valuable product (Cegarra J. et al., 1996a; Paredes C. et al., 1996a,b, 1998, 1999a, 2000, 2001 and 2002; Filippi C. et al., 2002). Furthermore, it produces no liquid waste, has a low fixed cost and the final product could be marketed as a high-quality soil conditioner (Vlyssides A.G. et al., 1989). A drawback of composting is the fact that the quantity of (semi-) solid olive-mill wastes is not sufficient for all the waste produced and hence either an additional woody substrate or condensation of the waste is required. In the latter case the condensation of the toxic compounds from the waste will hinder the process of composting and decrease the soil-enhancing quality of the final product. Another drawback is the high increase of pH produced during the composting of olive-mill wastes (Cegarra J. et al., 1996a; Paredes C. et al., 2000) which may limit its agricultural use, not only when used as soil-less substrate but also as soil amendment in high pH soils. The addition of elemental sulfur during the maturation phase of the composting process was considered a recommended method for decreasing the pH of the composts under the organic agriculture regulations — see Appendix II of EEC Council Regulation 2092/91, where elemental sulfur appears as an allowed soil fertilizer (Roig A. et al., 2004). The decrease of the pH reflected the formation of H+ as result of sulfur oxidation. Sulfur is oxidized to H2SO4 by sulfur-oxidizing microorganisms (actionomycetes and filamentous fungi and Thiobacillus bacteria) according to the following mechanism:

2POMW has been composted by Sciancalepore V. et al. (1994, 1995, 1996). The quality of cured compost obtained by a mixture of crude olive cake, 2POMW and fresh olive tree leaves inoculated with cow manure, after six months of composting has been evaluated. The composting process brought about the total disappearance of phytotoxicity encountered in raw materials. The development of enzymatic activities was positive and no pathogen was found. The compost can, therefore, be satisfactory used as amendment for agricultural crops.

Roig A. et al. (2004) studied in a laboratory scale incubation experiment the effect of different variables (moisture, temperature, and sulfur concentration) on the oxidation rate of elemental sulfur, added to an organic compost prepared with 2POMW and sheep litter. An addition of 0.5% in sulfur (dry weight basis) and a moisture content of 40% were proposed as the optimum conditions to decrease the compost pH by 1.1 units without increasing the electrical conductivity to levels that could reduce the agricultural value of the compost. Compost treated with elemental sulfur did not show any potential phytotoxic effect as far as germination index is concerned. Although temperature was not an important factor for the oxidation rate, the control of moisture was considered to be decisive for the correct development of the process.

A number of laboratory studies assessed the suitability, as a vermicomposting substrate, of exhausted 2POMW either alone or mixed with cattle manure and/or municipal biosolids (Nogales R. et al., 1999). Cattle manure alone was used as a substrate for comparison. Five earthworms (Eisenia andrei) were added to 300 g of substrate and incubated for 17 weeks. Substrates examined were: exhausted 2POMW, cattle manure (CM), mixtures of exhausted 2POMW and cattle manure (2POMW:CM 8:1, 2POMW:CM 2:1), mixtures of exhausted 2POMW and biosolids (2POMW:BS 16:1, 2POMW:BS 8:1) and a 16:1:1 mixture of 2POMW, manure and biosolids (2POMW:CM:BS). Where biosolids were added, a preincubation was required to remove substances toxic to earthworms. All substrates supported earthworm growth and reproduction, with growth occurring for 4-8 weeks. Earthworm growth was considerably greater in the manure only substrate than in the exhausted 2POMW only substrate. The addition of manure or biosolids to the exhausted 2POMW enabled similar earthworm growth to that in the manure only. After 17 weeks, the earthworms inoculated at the beginning of the experiment had similar biomass, in all substrates. Larger weights of newly hatched earthworms were obtained in the substrates containing exhausted 2POMW. For a range of reproductive parameters including, among others, cocoon production and hatching success, all substrates were satisfactory with the 2POMW:CM 8:1, 2POMW:CM 2:1, and 2POMW:CM:BS being the most favorable for reproduction. For all substrates with 2POMW, vermicomposting reduced the organic carbon content, appreciably reduced the C:N and reduced the pH. A bioassay indicated that the final products were not phytotoxic.

A further study examined the feasibility of vermicomposting to stabilize exhausted 2POMW, for use as a soil amendment, using cattle manure (CM), anaerobic sewage sludge (ANS) and aerobic sewage sludge (AES) co-composting agents (Nogales R. et al., 1998; Sainz H. et al., 2000). Different ratios of 2POMW to co-composting agent were examined. Earthworm (Eisenia andrei) growth, clitellum development, and cocoon production were monitored over 35 days. Exhausted 2POMW alone was an inadequate substrate for vermicomposting on account of slow earthworm growth and infertility. The most effective ratios were: 2POMW:CM of 2:1 and 1:1, 2POMW:ANS of 16:1, 12:1 and 8:1, and 2POMW:AES of 16:1 and 12:1. Vermicomposting for 35 days reduced the dry weight of the substrates by 21-28%, and appreciably decreased their C:N. All final products had low contents of heavy metals. The above study demonstrated that exhausted 2POMW is a suitable medium for vermicomposting when combined with N-rich materials such as cattle manure and sewage sludge in appropriate ratios.

The characteristics of 2POMW are an obstacle for its correct aeration as a composting substrate, because such a process must be carried out in favorable conditions (appropriate moisture, nutrient balance, structure, and air distribution) to obtain a useful product (Alburquerque J.A. et al., 2004).

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