Combustion Characteristics of Sewage Sludge and Algae

Sewage sludge is a good source of energetically desirable compounds and green algae in developing country like India. In this work, the combustion characteristics of sewage sludge and green algae are described. The calorific value (CV) of dried sewage sludge (n = 7) and green algae (n = 11) was ranged from 1800 to 6250 and 3700 to 6280 kcal/kg with mean value of 3999 ± 1347 and 4428 ± 547 kcal/kg, respectively. The effect of proximate variables and water quality parameters in the CV of sludge and algae are discussed.


Introduction
Sewage sludge includes black and grey water and semi-solid precipitates released at the household levels.Total sewage generated from urban centres in India is ≈4.0 × 10 10 L/day.The sewage sludge is a source of nutrients and pathogenic microbes.The sewage sludge is covered by the algal bloom in urban sectors of India.Algal bloom causes tastes and odors in the water.Some algae produce an aromatic odor resembling to that of particular flowers or vegetables.In addition, a spicy, a fishy odor and a grassy odor can also be produced by algae [1].Sewage is a source of biomass energy that is very similar to the animal wastes.The caloric value of the sewage and algae was reported by many researchers [2]- [11].In this work, the caloric value of sludge and algal bloom of sewage reservoirs of Raipur city, CG, India is described for their further uses in energy generation.

Study Area
Raipur is a capital city of Chhattisgarh state of India, growing rapidly in unsustainable order due to being commercial center for steel, cement and forest products.At least two million habitants are living in this city over ≈1.0 × 10 3 km 2 area by discharging ≈3.0 × 10 7 L/day waste water.They are dumped into seven open reservoirs constructed in the outer of city, Figure 1.The area of sewage reservoirs is ranged from 2 -4 km 2 .Among them, two reservoirs i.e.Urla and Siltara lie in the industrial area.

Sample Collection
The composite water sample (100 × 5 mL from five points of each location) was collected into glass bottles (500-mL) in January 2016 as prescribed in the literature [12].The floating algal biomass was collected manually by use of knife in a 1-L glass beaker from the waste water by subsequent washing with deionized water thrice.One kilogram of the top composite sewage sludge from five points of each location (0 -10 cm) was taken from seven reservoirs in January 2016 by use of a stainless steel spoon and stored in a glass jar [13].The sewage sludge and algal biomass were dried for 2 days.Thereafter, they were dried in an oven for 2 days at 50˚C to remove the moisture.The dried mass was crushed and sieved out particles of <0.25 mm for the combustion studies.

Analysis
The physical parameters i.e. temperature (T), pH, electrical conductivity (EC), dissolved oxygen (DO) and reduction potential (RP) values of the water were measured at spot by Hanna made sensors.The total hardness (TH) and total alkalinity (TA) values of the filtered water were determined by the titration methods in the laboratory.The F − content of water was analyzed by using the Metrohm-720 ion selective electrode in the presence of the buffer.The content of ions i.e.Cl − , 3 NO − , 2 4 SO − , 4 NH + , Na + and K + in water and sludge were analyzed by Dionex-1100 ion chromatography.A 0.25 g of the sludge sample was digested with acids (3 mL HCl and 1 mL HNO 3 ) in a closed system with P/T MARS CEM (Varian Company) microwave oven for use of metal analysis.The metals (i.e.Cr, Fe, Mn, Ni, Cu and Zn) in the extract were analyzed by the Varian ICP-OES-700-ES.
The bulk density (BD) was measured by the water replacement method [14].The moisture content of the tree samples were analyzed by heating it at 105˚C ± 2˚C till constant weight is observed.The ash content of the materials was determined by heating the sample at 600˚C for 4 hr [15].The calorific value (CV) of the sample was determined experimentally using a Digital Microprocessor Based Bomb Calorimeter UTS 1.34, Advance Research Instruments Co., New Delhi as described in the literature [16].The calorific value was calculated by using the following equation: where, GCV, M, We, W 1 , W 2 and ∆T represent gross calorific value of species (kcal/kg), water equivalent, weight of sample, weight of cotton thread, weight of fuse wire and rise in temperature, respectively.The content of volatile compounds including moisture was analyzed by using a Metller thermogravimetric analyzer-TGA-2 at heating rate of 20˚C/min.
The value of non-volatile (fixed) carbon (FC) was computed by subtracting the fraction (%) of moisture (MC), volatile compounds (VCs) and ash residue (AR) to the 100.

Results and Discussion
Algae are main primary byproducts in all kinds of water bodies due to eutrophication, and being indicators of water pollution in a number of significant ways due to availability of contained nutrients.Algae are among the fastest-growing plants in the world and have much faster growth-rates than terrestrial crops.All sewage sludge reservoirs of country like India are floated with different types of algal blooms due to variations in the topology and exposure extent of sun light and nutrient levels.

Combustion Characteristics of Algae
The combustion characteristics of four algae i.e. hydrodictyon, lyngbya, microcystis, and phithophoragrown in seven reservoirs is shown in Table 5.The value of BD, MC, CV and AR was ranged from 910 -1070 kg/m 3 , 1.5% -4.6%, 3700 -6280 kcal/kg and 1.4% -3.8% with mean value of 963 ± 32 kg/m 3 , 3.4% ± 0.6%, 4428 ± 587 kcal/kg and 2.8% ± 0.5%, respectively.The higher CV for hydrodictyon and lyngbya algae was observed, may be due to higher BD value, Figure 4.The CV of algal bloom was correlated well with BD unlikely to the MC and AR, Figure 5.The TGA chromatogram for hydrodictyon algae is shown in Figure 6.The light and heavy volatile compounds (VCs) content of the dried hydrodictyon algae was found to be 5.2% and 27.4%, respectively.The nonvolatile (fixed) carbon and ash residue content of the algae was computed and found to be 64.8%.The CV of the dried sewage sludge and algal biomass of the studied area was found to be comparable to other regions of the World [2]- [11].

Conclusion
The calorific value of the sewage sludge is found to be similar to biomass and animal waste.The heat value of the sludge and algae was found to be comparable.The heat value of algal boom is seemed to be independent of quality of the sludge because it is produced by process of eutrophication.

Figure 1 .
Figure 1.Representation of location of sewage ponds in Raipur city, India.

Figure 2 .
Figure 2. Spatial variation of concentration of elements in sludge reservoirs.

Figure 3 .
Figure 3. Correlation of nutrient content, bulk density (BD), moisture content (MC) and ash residue (AR) with calorific value of sewage sludge.

Table 1 .
Physical characteristics of sewage water.

Table 2 .
Chemical characteristics of sewage water, mg/L.

Table 3 .
Energy characteristics of dried sewage sludge.

Table 4 .
Correlation matrix of water parameters and CV.