Composting test

The composting was conducted using pilot-scale composting facilities (Figure 1), the details of which are shown in Kuroda et al [10]. The main part of the facility was a tank with an inner capacity of 1.8 m³ (1 m in width and depth, 1.84 m in height), consisting of an outer shell (stainless-steel panel) and an inner filling for insulation (polystyrene board). The tank was filled with the composting material, and during composting, the material was continuously aerated using an air pump connected to the bottom of the tank. Thermocouple sensor rods were inserted into the tank to monitor the temperatures of the composting material during composting.

Scheme of composting facility. The unit of measures in the figure is cm.

Table 1 lists the settings of the composting test. Composting mixtures were prepared from dairy cattle manure and sawdust by mixing a weight ratio of 5:1. Two treatments were set: WCO was added to the mixture at 0 wt% (Control) and 3 wt% (WCO3) of the manure. The mixtures were placed in the tanks and continuous aeration was supplied at a flow rate of 40 L/min, corresponding to 22.2 L/(min·m³) of the initial mixture in the tank at the start of composting. This aeration rate was close to the low aeration rate (21.4 L/(min·m³) of the initial mixture) at which reduction of NH₃ emissions was observed after WCO addition in the laboratory-scale composting tests in our previous study [9]. It was substantially lower than those applied to practical-scale composting treatments of animal manure, 50 to 300 L/(min·m³) of the composting material, and usually ≥100 L/(min·m³) of the material [11].

Composting test parameters

WCO, waste cooking oil.

The composting period was set to 28 d. During compost ing, exhaust gases were collected from the exhaust pipes connected near the top of the tank, and NH₃ concentrations in the gases were measured using a detection tube (No. 3L or 3M; Gastec Co., Ayase, Japan) at intervals of 12 or 24 h. On days 7, 14, and 21, the mixtures were removed from the tanks, mixed completely, and placed in the tanks again (turning). At every turning, the mixtures were weighed before and after turning, and 3 kg of each mixture was collected after turning. The collected samples were analyzed using various analytical methods described below. Using the same settings, the composting test was repeated three times.