A Study On The Technique For Culturing Marine Microalgae With Sewage Sludge From Municipal Wastewater Treatment Plants

In recent years, with the improvement of ability in municipal wastewater treatment, more and more sewage sludge has also been produced in our country. Except high moisture content, sewage sludge contains many other ingredients, including not only massive nutrients such as N, P, K, trace elements, but also harmful substances such as heavy metals and organic pollutants which are difficult to decomposed. Direct discharge or improper management will lead to the pollution of water body. At present, there are two major problems including high cost and low production in the cultivation of marine microalgaes, which cause a lower industrial microalgae production than the theoretical calculation. Cultivation of marine microalgae using sewage sludge, not only may save the massive culture medium by providing inexpensive raw material, but also can accelerate the reclamation of sewage sludge by preparing single cell protein feed and the industrial chemicals with microalgae. Unfortunately, less research about this has been undertaken. Therefore, it is very important to study the technique for cultivating marine microalgae and producing single cell protein using sewage sludge.In this study, two methods, pestle followed by centrifugation(PC) and alkali boiling followed by centrifugation(ABC), are selected to extract the nutrients from sewage sludge discharged by a municipal wastewater treatment plant. Then, a series of culture solution are prepared by mixing the sludge extraction fluid with the traditional culture medium in different proportions to cultivate two kind of marine microalgae, Chlorella Spp. and Spirulina platensis. The suitable extract method and the optimal mixing proportion are obtained through the comparison of microalgae biomass in various mixtures. On the basis of this, the environmental factors which affect microalgae growing in the mixture are optimized. After 6 days, the maximum biomass of Chlorella Spp.and Spirulina platensis at the most suitable conditions is determined. At last, the nutrition and security of microalgae cultured by the sludge extraction fluid are evaluated. The main conclusions are listed as follows: (1)Both Spirulina platensis and Chlorella Spp.grow well in the mixture in which sludge extraction fluid has taken the place of traditional culture medium partly. The sludge extraction fluid with PC method is more suitable for microalgae growth than that with ABC method. Moreover, PC method can be operated easyly and rapidly. Appropriate proportion by which the sludge extraction fluid mixed with traditional culture medium should be determined depending on the varity of microalgae, because they are adapted for in a certain content range of nutrients in the mixture. For the cultivation of Chlorella Spp, sludge extraction fluid and F/2 culture medium should be mixed in a volume ratio of 7:3, while, for the cultivation of Spirulina platensis, sludge extraction fluid and Zarrouk culture medium should be mixed in a volume ratio of 5:5 or 6:4.(2)In the mixture that the volume ratio between sludge extraction fluid and F/2 culture medium is 7:3, the best growth for Chlorella Spp. is observed at 25℃and pH6.5, illumination time of 18h, illumination strength of 500lx. In the mixture that the volume ratio between sludge extraction fluid and Zarrouk culture medium is 6:4, the best growth for Spirulina platensis is observed at 35℃and pH8.5, illumination time of 18h and illumination strength of 3500lx. During six days, both Chlorella Spp.and Spirulina platensis can absorb inorganic nutrients from the culture mixture quickly under the best growth conditions. Chlorella Spp.can remove 97.32% of PO43--P, 96.72% of NO2--N, 97.55% of NH4+-N,78.13% of NO3--N and 65.88% of COD in the mixture respectively, while Spirulina platensis can remove ratio 98.06% of PO43--P, 89.35% of NO2--N, 98.39% of NH4+-N, 98.33% of NO3--N and 72.58% of COD respectively. After cultivation for 6 days, the mixtures contain only a few nutrients and COD which are up to the Integrated Wastewater Discharge Standard of China, and can be discharged directly without pollution in the water.(3)Under the best growth conditions, the biomass of microalgae cultured in the mixture is higher than that cultured in tradtional culture medium. While, protein content in microalgae cultured in the mixture are almost the same as that cultured in the tradtional culture medium. Microalgae cultured with sludge extraction fluid contains seven kinds of essential amino acids and eight kinds of inessential amino acids, which contents are almost the same as that cultured with the tradtional culture medium. In addition, the former also has Lys and Met which are not found in the feed from plants. Microalgae cultured with sludge extraction fluid has less heavy metals than the standard of The Hygeian Index of Feed and Additive and some other standards. Therefore, these microalgae products have very good nutrition and security when using as protein feed or feed additive.In conclusion, cultivation of Chlorella Spp.and Spirulina platensis with sewage sludge from municipal wastewater treatment plants, not only can reduce the cost for microalgae production largely, but also are beneficial to the reclamation of sewage sludge. Being as the protein feed or the feed additive, the microalgae products is sure to relax the tension of animal feed effectively. This way for sewage sludge utilization is in conformity with the principle of sustainable development and circular economy for its economic and environmental benefit, and thus has a bright future.