Abstract
Huge quantities of palm tree residues are available annually in Saudi Arabia; they are often disposed of by burning, leading to large environmental pollution and health problems. Enclosed composting may be a successful method for utilizing these residues. Effective composting in a rotary bioreactor depends on the composting materials and the frequencies of the rotation–aeration process. Here, we attempted to determine the optimal method for composting these residues in a bioreactor. Four identical pilot-scale bioreactors (D1–D4) were used. The respective aeration–rotation frequencies were continuous (D1), and 20 min per 1 h (D2), 2 h (D3), and 3 h (D4); the rotation speed of the bioreactors was 3 rpm. Three periods were analyzed; their names and their respective lengths for D1, D2, D3, and D4 were as follows: the lag period was 10 h for each, the mesophilic periods were 13, 14, 89, and 231 h, and the thermophilic periods were 0, 55, 71, and 17 h. The peak compost temperatures (Tc, max) for D1, D2, and D3 were 65.2, 57.2, and 46.9 °C, and were achieved at 36, 71, and 108 h of operation, respectively. In D1, Tc quickly declined after reaching Tc, max due to limited microbial activity at Tc > 60 °C. In D2, a Tc of 50–57.2 °C was maintained for 61 h, and the highest organic matter degradation (OMD) of 0.30 was achieved. During the composting period, the carbon to nitrogen (C/N) ratio and moisture contents (MC) remained in the optimal ranges for microorganisms and did not affect the composting process. The results indicated that combined aeration–rotation for 20 min every hour (D2) gave the best results for composting palm tree residues, it ensured the hygiene safety of the end product, and it also consumed less power for aeration and rotation than D1.
Highlights
Date palm (Phoenix dactylifera L.) retains a distinguished position in many countries in the world, in the Middle East area and in the Arabian Peninsula
Four operation methods were suggested and evaluated for composting palm tree residues mixed with chicken manure (1:0.75 w/w) in rotary bioreactors
Nitrogen and moisture content (MC) did not affect the bioreactor performance as well as the composting process because both the carbon to nitrogen (C/N) ratio and MC were in favorable ranges during the composting time
Summary
Date palm (Phoenix dactylifera L.) retains a distinguished position in many countries in the world, in the Middle East area and in the Arabian Peninsula. Enclosed composting systems (rotary bioreactors) have proven to be an efficient technique in farm-scale composting or small projects, and are the most suitable composting systems to be used successfully in the Arabian Peninsula [11]. This is because of the dry climate, high daily water evaporation rate, and water shortage [12]. Research groups have investigated the number of rotations in a 24 h period: one rotation [15], two rotations [17], three rotations [18], and four rotations [16] In these studies, the composting materials were vegetable wastes and tree leaves. Continuous aeration–rotation was used for composting tomato plant residues mixed with chicken manure in a pilot-scale bioreactor with the aim of reducing the active phase period [11] and analyzing energy performance [24]
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