| Owning to the rapid development of pig husbandry, there is a dramatic increase in the swine wastewater, which brings a severe damage to the local ecological environment and health of the residents. It is a foundation for the sustainable development of the pig husbandry to realize the innocent, resourceful and reduction treatment of the swine wastewater. However, the current regular wastewater treatment technologies only focus on the innocent treatment of the wastewater, but neglect the resourceful utilization of the wastewater. On the other hand, in order to alleviate the situation in which there is a shortage of resource and energy, the microalgal bio-energy turns to be one of the hot spots in the research field of biomass energy, and how to lower the cost for microalgal cultivation is a key to realize the commercial development of the microalgal biofuel. The occurrence of wastewater treatment technology based on the microalgal cultivation makes it possible to realize the innocent treatment and resourceful utilization of swine wastewater and to lower the cost for microalgal cultivation. It couples the water purification with the production of valuable biomass, which realizes the transformation of wastewater treatment from treatment process to production technology, and the microalgal biomass obtained can act as the raw materials for producing the biodiesel, which can create economic benefit and guarantee the sustainable development of the pig farm. However, the selection of algae and construction of cultivation system are the keys to realize the treatment technology successfully. In this experiment, a microalgal in the wastewater treatment tank of the pig farm was separated and purified. Therefore, the project focused on the discussion about the isolated cultivation of microalgal and the basic conditions for its application in the swine wastewater treatment, which provided the information basis for the swine wastewater treatment technology based on the microalgal cultivation. The research contents mainly included the isolation and identification of the microalgal and determination of its physicochemical characteristics, the optimization of the microalgal cultivation conditions, feasibility of swine wastewater treatment based on microalgae as well as the strengthening in the condition of the microalgal swine wastewater treatment, etc. The following results have been obtained through experimental research:(1) With the help of morphology and molecular biology approaches, this microalgal was identified as the Scenedesmus and named as Desmodesmus sp. CHX1. The related purified strain and gene sequence has already been submitted to the China General Microbiological Culture Collection Center (Accepted No. of the preservation: CGMCC No.6649) and Genebank (accession number:JX255841). The best optical density of the Desmodesmus sp. CHX1was OD690, but the measured value of OD690could not exceed0.7easily. Within this measuring range, there was a linear positive correlation between the optical density and the dry weight of the cells (R=0.9942). Under normal cultivation, the Desmodesmus sp. CHX1reached stable growth phase after growing in the BG-11culture solution for45days, with the maximum density4.54g/1. After four weeks, the contents of the C, N, H and Zn in the cell of Desmodesmus sp. CHX1were45.7%,7.13%,7.90%and79.7mg/kg, respectively, and the content of Cu was lower than the minimum detectable concentration. The total Chlorophyll was4.92mg/l, in which the Chlorophyll a accounted for78.5%of the total chlorophyll, while the content of carotenoid was1.67mg/l. The total lipid of the isolated alga was the range of12.1%-14.6%, and the content of C16-C18dominated in the fatty acid, accounting for70.7%of the fatty acid content, which could be used as the raw materials for the biodiesel. In addition, the crude protein content was49.3%, and there were7kinds of amino acid which were essential for the human body among the16kinds of amino acid, in which the content of aspartic acid was the highest, about27.2mg/g; the content of total amino acid was198.4mg/g, in which the content of essential amino acids were59.31mg/g, accounting for29.9%of the total amino acid. Also it could be taken as the substitutes of some protein feed in the pig farm.(2) The mixotrophic cultivation was favorable for the growth of Desmodesmus sp. CHX1most. When the glucose and NO3-N were taken as the carbon and nitrogen source, respectively, it was favorable for the biological yield of the Desmodesmus sp. CHX1and the accumulation of Chl (a+b). The orthogonal optimization test had shown that within the four factors measured, the additive concentration of glucose had the biggest influence on the growth and oil production rate of the Desmodesmus sp. CHX1and the initial inoculation concentration ranked the second. According to the results of orthogonal test and single factor experiment, the combination of cultures that was most favorable for the growth and oil production rate of the Desmodesmus sp. CHX1had been selected, namely, the concentration of glucose was3g/1, the initial pH of the medium was10, the cell inoculation concentration was5%(initially, OD690=0.08) and the concentration of the nitrate was2g/1. In such a culture, it only took5days for the Desmodesmus sp. CHX1to reach the stable phase from the end of adaptive phase, which shortened the cultivation period greatly. As a result, the culture of Desmodesmus sp. CHX1seed could be prepared in the optimized cultivation condition. In addition, the aeration ccould increase the biological yield of the Desmodesmus sp. CHX1, the content of Chl (a+b) and oil production dramatically. Within the research scope (16-1601/h), the above mentioned index increased with the aeration quantity. After7days of cultivation, the biomass concentration of the Desmodesmus sp. CHX1cell, the Chl (a+b) content and oil production reached7.26g/1,60.5mg/1and128.7mg/l/d, respectively, when the aeration was1601/h, suggesting that during the cultivation of Desmodesmus sp. CHX1, proper aeration is favorable for the growth of microalgae and the accumulation of valuable additional materials.(3) Desmodesmus sp. CHX1could survive and grow in the properly-diluted swine wastewater after sterilization. After12days of cultivation, the maximum biomass yield and growth rate reached196.7mg/l/d and0.279/d, respectively, while the maximum removal rate of NH4+-N, TP and COD reached98.2%,80.4%and37.1%, respectively. Therefore, Desmodesmus sp. CHX1has the potential of being applied into the swine wastewater. But before treatment, proper dilution of swine wastewater is a premise of guaranteeing the fast growth of Desmodesmus sp. CHX1and effective removal of nutritional ingredients. However, in the current commercial operation model, dilution is limited in the commercial application. Therefore, to decrease the inhibiting effect of swine wastewater on Desmodesmus sp. CHX1, other low-cost pre-treatment methods can be sought to strengthen the feasibility of its commercial application.(4) Direct air stripping could be taken as a simple and effective pre-treatment applied in the swine wastewater treatment technology system based on the microalgal cultivation to decrease the inhibition of high ammonia and nitrogen in the swine wastewater on the microalgal. Besides, to pump the air with5%CO2(namely as the mixotrophic cultivation) into the unsterilized swine wastewater, the growth of Desmodesmus sp. CHX1, the accumulation of oil and the removal rate of NH4+-N, TN, TP, COD, Cu and Zn in the wastewater were strengthened remarkably. After8days of cultivation, the biomass concentration, biomass yield and oil production rate of Desmodesmus sp. CHX1were as high as7.91g/1,869.0mg/1/d and132.1mg/l/d (the content of the total oil is15.2%), while the removal rate of NH4+-N, TN, TP, COD, Cu and Zn in the wastewater was97.3%,87.9%,93.2%,41.8%,50.1%and30.1%respectively. The elementary stream analysis results had shown that, direct absorption and utilization of Desmodesmus sp. CHX1were the main causes of the removal of N and P, as well as Cu and Zn in the wastewater. After the mixotrophic cultivation, the content of N, P, fatty acid content of C16-C18, crude protein content and amino acid content in the cell of Desmodesmus sp. CHX1were improved greatly, in which the content of N and P was9.55%and0.79mg/kg respectively, while the crude protein content and total amino acid content are59.7%and315.9μg/g, respectively (EAA/TAA is30.5%), and the fatty acid content of C16-C18accounted for83.11%of the total fatty acid. |
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