Application of sludge in preparation of artificial algal reef

Abstract: There is abundant resource of tidal flat rich in nutritive material along the coastline of China. Except part of sludge and sandy tidal flat has been exploited for shellfish aquaculture, there is still a lot of sludge tidal flat that is rarely to be utilized. Sludge tidal flat is rich in organics, such as nitrogen and phosphorus. If the nutritive materials in sludge tidal flat are effectively combined with the multiplication process of artificial algal reef, the construction process of artificial sea-grass field will be certainly accelerated. For the effective utilization of the nutrient in sludge tidal flat, the sludge was selected as the investigated subject dug from the Shuangdao bay in Weihai City, Shandong Province. The raw materials of sludge were extracted by the way of sifting and drying, and the algal reef blocks were prepared according to 4 ratios of sludge to cement. A linear regression was adopted to analyze the elution effect of nutrient salts from the algal reef blocks, and the compression strength of reef blocks were measured as well. The results showed that among different ratios of sludge to cement, the elution rules of nitrogen and phosphorus from the reef blocks were basically accorded with the linear mode, for which the fitting degrees were all greater than 0.73. The elution effect of nutrient salts were all accorded with the release rule of slow-release fertilizer, which could continuously elute the nutritive materials containing nitrogen and phosphorus into the water to provide the necessary nutrient for the growth of algae. The sludge content of reef blocks was proportional to the elution effect of ammonium salt, and there was significant difference among 4 ratios of sludge to cement (P<0.01); the sludge content was also proportional to the elution effect of phosphate, while there was no significant difference (P>0.05), but the phosphorus contents eluted from the ratio of slug to cement of 0.9:1 and 0.7:1 were higher than the ratio of 0.5:1 and 0.3:1; in addition, the sludge content of reef blocks was inversely proportional to the compression strength of reef blocks which achieved the concrete strength standard from C15 to C25. In order to strengthen the multiplication effect of artificial algal reef, the ratio of sludge to cement can be adjusted to fulfil the demand of the ratio of nitrogen to phosphorous in water, which can increase the attachment and survival probability of algae on the artificial reef. The reef blocks in this paper have been built only by sludge and cement, which are not the usual materials of concrete. If some percentage of stone and sand are mixed into the manufacture process of these reef blocks, their strength will be promoted greatly. Therefore, the sludge tidal flat is expected to partly replace the usual concrete materials in the future during the manufacture process of artificial algal reef. This research can provide theory basis and technology reference for the application of sludge tidal flat in the manufacture and multiplication of artificial algal reef.