Slurry form swine (feeder to finish) wastewater was collected from the university farm and kept at 4°C until used. For separation of the solids from wastewater, the slurry was flocculated with 100 times diluted polymer (commercial organic polymer, Nalco 855) at a rate of 50 mg/L, and then the mixture was stirred for 30 min and kept for 1 h for settlement. The separated solids from the swine wastewater were dried in an open plastic vessel for 3 d under natural conditions (i.e., in an experimental shed) for the evaluation of the solid waste to be recycled as a solid fuel.

The wastewater collected after solid separation was analyzed for total solid (TS), suspended solids (SS), ammonia-nitrogen (NH₄-N), orthophosphate (OP), total Kjeldahl nitrogen (TKN), total phosphate (TP), and total soluble organic carbons (TOCs). NH₄-N, OP, TKN, and TP were analyzed with an auto water analyzer (Quick Chem 8500, LACHAT), and TOCs wereanalyzed using a Total Organic Carbon Analyzer (Shimadzu, TOC-5000A). The separated solid after natural drying was measured for moisture, TS, volatile solid (VS), ash and energy level. For estimation of moisture content, the solids were dried in electric oven at approximately 105°C for 24 h. For VS and ash, the weight of solids was measured before and after ignition at 550°C for 4 h and the VS being the difference between the dried solids and the ash. All the analyses were carried out according to APHA (2005) in triplicate. The calorific value of the solid waste was measured by burning the weighed (0.5 g) sample in an oxygen bomb calorimeter according to ASTM E-711.

A combustion experiment was done with a small boiler furnace consisting of fine grid body, fuel supply, combusting chamber and the ash pan. The furnace had an area of 66 to 132 m². Boiler dimensions (W×L×H) were 650×1,230×1,600 (mm) with a capacity of 110 L for liquid and 210 kg for solid. Operating conditions during combustion are given in Table 1. The analysis of the flue gas composition was carried out immediately using “Combustion Gas Analyzer” (ECOM Ltd America, ECOM – AC 2.), and a multi-component gas analyzer was used for online measurements of CO, NO, SO₂ and O₂. Table 2 shows the ranges for the different gas components. The microprocessor calculated CO₂ and combustion efficiency.

The separated solids from swine wastewater were subjected to TG and DT analysis in nitrogen and air atmosphere using STA 409 PC Luxx^® Simultaneous TGA-DTA analyzer, where the mass loss, TG, and temperature changes were recorded simultaneously. Thermogravimetric curves were obtained at a heating rate of 10°C/min. Purified nitrogen and air at a flow rate of around 50 ml/min was used as the purge gas to provide an inert/oxidative atmosphere for pyrolysis and combustion, respectively. Mineral analysis of the solid and ash were examined through the optical emission spectroscopy.

The main physio-chemical parameters which determine the potential recovery of energy from solid wastes are density, moisture content and calorific value (Houshfar et al., 2010). Wastes of high density reflect a high proportion of biodegradable organic matter and moisture. High moisture (>45%) makes the waste rather unsuitable for thermo-chemical conversion for energy recovery as heat must first be supplied to remove moisture. The results obtained in the present study are shown in Table 4. The calorific value of the solid is much higher (5,330.40 kcal/kg) compared to those of horse manure mixed with wood shavings (4,626.44 kcal/kg) having moisture content of 57% (Lundgren and Pettersson, 2009), poultry litter having moisture content of 15.02% and calorific content of 1,440.24 kcal/kg (Zhu et al., 2005) and feedlot manure having 4,991.87 kcal/kg calorific value (Sweeten et al., 2003). Therefore, it appears that the sludge or solids produced from the swine wastewater treatment process y using an organic polymer could be a useful source for energy generation. The TS, organic and inorganic content were found to be 84.61, 67.10 and 32.89%, respectively.

Both CO and NOx emissions depend upon matter characteristics, operating condition temperature and excess air etc. CO and NOx fluctuate more with higher sludge solid content (TS) (Dangtran et al., 2000). Figure 1b shows that with an increase in temperature from 100 to 240°C, there was an increase in CO (600 to 880 ppm) and NOx (375 to 420 ppm) emissions. In the case of CO, an increase in emission was observed in steady intervals at 140 to 160 and 180 to 200°C, whereas in the case of NOx increased emission was observed till 180°C and thereafter emission was stable. Gases emitted from the combustion chamber are mainly NO₂ and NO, which are collectively defined as NOx. NOx emission can be categorized as thermal NOx, prompt NOx and fuel NOx. During gas fuel or denitrified fuel combustion, thermal NOx and prompt NOx are produced. If nitrogen content is >3% as in case of heavy oil or charcoal, then fuel NOx is main cause of NOx production. In this experiment, NO produced from the solids was less than 400 ppm which is comparatively lower than the standard environmental value (500 ppm).

The slight increase in NOx with increase in temperature could be due to the release of fuel-N from NH₃ groups and thus enhanced formation of NOx coming from a small percentage of nitrogen bound in the sludge (Zhu et al., 2005). Similar results were also obtained by Zhu and Lee (2005) while assessing temperature effect on NOx emission of sawdust, poultry litter and manure co-combusted with natural gas.

From Figure 1a, it can be noticed that air excess ratio was increasing with the increase in temperature. With increase in the combustion temperature, CO concentration value was observed to be high (Figure 1b). CO concentration in flue gas heavily relies on air fuel ratio (equivalence ratio; Ø). With high O₂ content there is complete oxidation of CO to CO₂, but with a lack of O₂ a large amount of CO is produced. In other words, if Ø >1, there is significant increase in CO emission conversely if Ø <1 only small concentration of CO is formed. Hence, for low CO emission a high air-fuel ratio is required.

The CO₂ concentration was nearly 2%, and no noticeable change in concentration was observed with an increase or decrease of temperature as depicted (Figure 1c). The combustion efficiency was between 95 to 98%, suggesting the stable combustion in progress (Figure 1d).

TG analysis of the separated solid from swine wastewater at 10°C/min is presented in Figure 2.Concerning the TG data five thermal effects were clearly identified. The correlation between TG data and DTA curves contributed to the interpretation of these thermal effects. Four exothermal (in the temperature ranges 373.2, 459.1, 611.9 and 960.2°C) and one endothermal (107.5°C) peaks were determined during TG-DTA anlaysis. The peak at 107.5°C was caused by the volatilization of water in the solids (Iordanidis et al., 2001; Xiu et al., 2012). DTA curves indicate that it was an endothermic stage under inert conditions. Mass change of 1.20% was observed at this stage. Following this a huge mass change of 40.87% and 37.50% was observed between 200 to 400°C and 400 to 600°C, respectively. Consequently, 80% mass change was noticed in the temperature range of 200 to 680°C. The DTA curve displayed two exothermic peaks in this region, which is probably due to the decomposition reaction of the heavy components (hydrocarbons) in the solids (Xiu et al., 2012). After this intensive mass change, low mass changes of 1.80 and 1.87 were observed in the temperature range of 900 to 1,080 and 1,180 to 1,290°C, respectively. A total of 83.19% mass change was observed overall. On the basis of data obtained, the combustion of the separated solid from swine wastewater can be divided into three phases: In the first phase, water and light compounds volatilized and were oxidized, which occurred before 200°C; in the second phase, the heterogeneous combustion occurred between the heavy hydrocarbons and oxygen; this lead to the main weight loss in the second region. DTA curve analysis around 611.9°C showed an exothermic peak having a calorific value of 2,889.29 cal/kg. In the third stage, the char that was formed during the previous process of evaporating and cracking was observed by two peaks in the temperature of more than 900°C.

Optical emission spectrometric analysis was done to analyze inorganic components of the solid separated from swine wastewater and of bottom ash after combustion (Table 5). The analysis of the major inorganic components of the bottom ash compared with the solid showed decreased concentration of SO₃, Cl and K₂O in bottom ash, whereas substantial increase in MgO, SiO₂, and P₂O₅ was noticed after combustion.

Coal ash showed lower amount of Na₂O, K₂O and higher content of Al₂O₃, SiO₂ and Fe₂O₃ compared to horse manure and the bottom ash. Ca content of the bottom ash was higher than that of the horse fuel mixture and comparable to coal ash, thus can have higher capacity of self hardening due to formation of limestone (in humid conditions) on reaction with carbon dioxide. Nitrogen content is completely absent from the ash. The phosphorus content in the ash is high, and thus can replenish the lost phosphorus content of the soil. Since, heavy metals are an important issue with regard to assessing the quality of ash as possible fertilizer, the heavy metal content in the ash as well solid fuel was analyzed (Table 6). As and Se decreased in the bottom ash while Cd, Cr, Cu, Co, Mo, Ni, Pb and Zn increased. On comparing the legal limits stated by DuMV (2003), the ash contained much lower heavy metal content than the limit values. Sweeten et al. (2003) and Lundergen and Petterson (2009), reported that heavy metal contents in ash must be lower than the legal limits for its use as fertilizer or before being recycled to forests.