Elsevier

Ecological Engineering

Volume 116, June 2018, Pages 14-20
Ecological Engineering

Composting physics: A degradation process-determining tool for industrial sludge

https://doi.org/10.1016/j.ecoleng.2018.02.015Get rights and content

Highlights

  • Maximum temperature recorded during thermophilic phase is 45 °C.

  • Moisture content measured as 37% in the end of the composting process.

  • Moisture content and free air space was influenced by other physical parameters.

  • Regression analysis performed on bulk density and moisture content showed (R2 = 0.98).

  • Free air space and bulk density was positively correlated.

Abstract

Management of industrial sludge is a big challenge for developing countries. Composting is an economical and viable option to manage and treat an industrial sludge. However, efficient compost production requires nature of materials involved, process understanding and physics behind the process. We investigated the physical parameters during composting of solid pulp and paper mill sludge in 550 L rotary drum composter. Variation in physical parameters such as bulk density, volatile solids, moisture content, free air space, void ratio, ash content and particle density were analyzed over the period of composting. Bulk density was observed increasing, whereas free air space was showed declining trend and observed 52% in end compost. The particle density was observed to be increasing from 610 to 680 kg m−3. End compost was analyzed for nutritional parameters and seen to be increasing over the period of composting. A strong relationship was found between various physical parameters. Pearson’s correlation coefficient matrix was formed between free air space, bulk density, moisture content and particle density.

Introduction

The lignocellulosic waste is very difficult to manage as it is not easily degraded. Solid pulp and paper mill sludge (SPPMS) is one of industrial lignocellulosic biomass which has been utilized for compost production from the decade. SPPMS is generated as a result of pulp and paper mill effluent treatment. In general, per tonne of paper production generates 40–50 kg dry sludge and is a mixture of primary (70%) and secondary (30%) sludge (Bajpai, 2015, Hazarika et al., 2017). Owing to the presence of organic matter, the sludge produces greenhouse gases (CO2 and CH4) when disposed of in landfills. Availability of organic matter makes SPPMS a good substrate for composting. Composting process involves a natural biological decomposition of organic matter, which is carried out by naturally occurring microbes such as bacteria, fungi, actinomycetes thus converting into humus product i.e. compost (Pan et al., 2012). It is an entirely aerobic biological process that occurs under certain conditions, which allow development of thermophilic temperatures to produce compost that is free from pathogens and plant seeds and can be applied to land (Haug, 1993).

Several studies carried out on composting or vermicomposting of SPPMS showed improvement in nutrients concentration. A study carried out by Jackson and Line (1997) indicated an absence of phytotoxicity and reduction in pile volume by 45%, 31.6% increase in bulk density and 10% reduction in moisture content. The author further stated reduction of C: N ratio from 218:1 to 23:1 before mineral nutrient addition. The stability tests, the C:N ratio, and the nutrient-content during windrow composting of paper mill sludge from Virginia fiber corporation indicated that the compost was a high quality, fertile growth medium (Evanylo and Daniels, 1999). A study performed by Thyagarajan et al. (2010) on composting of pulp and paper mill industry sludge added with saw dust and cow dung revealed improvement in chemical and nutritional properties. Rotary drum composting of SPPMS indicated a reduction in bioavailability and leachability of toxic metals such as copper, iron, nickel, lead, chromium, zinc, mercury and manganese after 20 days composting period (Hazarika et al., 2017). However, such studies do not provide information about degradation process monitoring, air, and water distribution within the compost matrix, and composting physics. The composting study on SPPMS has been mostly carried out on understanding variation in heavy metals concentration, amount of gaseous emissions, and chemical and nutritional transformation during degradation process. However, composting physics behind the composting process have rarely been studied for SPPMS. The lignocellulose substrates such as SPPMS usually characterized by a slower attainment (>45 °C) of thermophilic temperatures thus slower degradation rate and longer duration of acquired stable compost. The lesser moisture content can have adverse impacts on microbial survival rate during the composting process. This research work is pointed at understanding the composting physics during degradation of lignocellulosic substrate by adding cow dung (inoculum) and saw dust (bulking agents), paying particular attention to its temperature change, moisture content reduction, and other physical parameters that controls entire composting process such as bulk density, porosity, free air space, void ratio and particle density.

Composting physics plays an important role during every stage of compost production, handling, and its utilization. It includes measurements of parameters such as bulk density, porosity, moisture content, free air space, void ratio, particle density within the compost matrix during the process. Besides food to microorganism ratio, composting is highly affected by water and air availability. To keep microorganism active, it is important to monitor and to provide optimum air and water during the process (Agnew and Leonard, 2003). Author further states that sufficient amount of moisture is required for microbial transport and nutrition. To support the metabolic processes of microbes, water plays an important role which is produced by and required for microbial activity (Baeta-Hall et al., 2005). Water is the medium for the transports nutrients, chemical reactions and allows the microbes to move about (Rynk, 1992). A range of moisture content reported by Jeris and Regan (1973) is 40–65% with a preferred range of 50–60% at the start of composting. The continuity of voids is also an important factor since this influences how easily air and water will flow through the material (Rynk, 1992). The heat and mass transport processes and therefore microbial kinetics in an organic compost matrix were influenced by free air space (Jeris and Regan, 1973, Miller, 1991, Haug, 1993). Bulk density was proving to be precious physical property as it determines power requirements for turning or mixing of wastes. According to Iqbal et al. (2010), a physical parameter holds a healthy relationship between each other, i.e., a small variation in one parameter may cause adverse effects on an additional parameter and thus degradation process would be compromised. No such studies have been carried out on giving the relationship between various physical parameters during composting of SPPMS.

Therefore, the current study focuses the composting of SPPMS added with cow dung and sawdust in the ratio of 6:3:1 as optimized by (Hazarika et al., 2017). The study aims to determine (1) how the combined addition of sawdust and cow dung affects composting physics during composting of SPPMS using rotary drum composter (2) how these additions improvise the chemical and nutritional quality of the final compost; and (3) correlations between various physical parameters.

Section snippets

Composting substrate and bulking agents

In the current study, industrial waste i.e. SPPMS were composted after added with sawdust and cow dung as inoculum. Experiments were carried out at Indian Institute of Technology Guwahati (IITG), Guwahati, Assam, INDIA. The substrate, SPPMS used were collected from Nagaon Paper Mill, a unit of Hindustan Paper Corporation situated in Nagaon, Assam, India. It is situated about 70 km from IITG campus. Fresh cow dung was gathered from a nearby dairy farm. Sawdust, which was used as a bulking agent,

Temperature and gravimetric moisture content

For the microbes to flourish and be most efficient in their activity, they should be provided with an appropriate nutrient, water, and oxygen supply, as well as environmental conditions in which thermophilic temperatures (greater than 40 °C) can be maintained (Agnew and Leonard, 2003). Thermophilic temperature improves the rate of reactions, kills harmful pathogens, and weed seeds during the composting process, hence, maintains sanitation capacity of the process (Haug, 1993). The current study

Regression analysis

Few physical parameters are correlated during the composting process. It was observed during composting of SPPMS those parameters such as free air space, moisture content and bulk density are interdependent on each other. The linear relationship was found to be the best relationship within these parameters. From Fig. 4 it is clear that direct correlation exists between free air space and moisture content shown (R2 = 0.97), whereas bulk density and moisture content exhibited (R2 = 0.98).

Conclusion

The moisture content determined as 37% at the end of the composting process. Less moisture in end compost indicated excessive compost matrix aeration during the composting process. Due to the presence of organic matter, the maximum temperature rise was observed up to 45 °C. Bulk density increased indeed from 306 to 436 kg m−3 owing to decrease in moisture content. A lesser amount of available moisture in end compost implies that higher availability of air within compost matrix. Results

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