Selection Of External Carbon Sources For Denitrification

Deficiency of carbon source for denitrification, which is very common, has been a significant factor limiting biological nitrogen removal in wastewater treatment plant in our country. Methanol is considered as a favorable external carbon source for denitrification. However, it has many disadvantages, such as high cost, high toxicity, and difficult in transportation. Therefore, other kinds of external carbon sources need to be explored and investigated. A program of selection and optimization of carbon sources was proposed in this paper based on the study of denitrification characteristics of several types of carbon sources.Through sequence batch denitrification experiments, the optimal C/N ratios of five external carbon sources, incluing methanol, acetic acid, propionic acid, butyric acid and glucose, were investigated, and the denitrification rates of these carbon sources were comparatively investigated under the condition of optimal C/N ratios. The results showed that the optimal C/N ratio of methanol, acetic acid, propionic acid, butyric acid, and glucose was 4.8, 4.9, 5.1, 5.5, and 7.1, respectively. Under the condition of optimal C/N ratios, the denitrification rate of methanol and acetic acid ranked the first, and propionic acid and butyric acid took the second place, while glucose was took the last place.When one type of carbon souce was changed to another, there needed a period of time for the denitrification bacteria to adapt it. For different type of carbon sources, the time interval of adaptation was different. The results showed that the denitrification system, when the carbon source changed from methanol to acetic acid, needed 35 cycles to achieve steady denitrification rate. When the original carbon source was changed to the former type again, the adaptation period was 59 cycles. When the caron source was changed from propionic acid to methanol, it took 69 cycles for the system to adapt. When the former carbon source (propionic acid) was used again, the adaptation period was only 33 cycles. It was faster for the denitrification system without external carbon sources to adapt to acetic acid than methanol: the adaptation period were 11 cycles and 61 cycles, respectively.To evaluate the feasibility of using food waste hydrolysates as carbon source, the infect of pH was investigated. Results showed that the intermittent adjusting of pH promooptimized hydrolyzing effect of food waste. When pH was adjusted to 7 or 8.5, it achieved optimal hydrolyzing effect, and after 7 days'hydrolysis, the effect reached to stabilization. When hydrolysates of food waste were used as external carbon source, it was found that when the C/N ratio was 9, it was difficult to achieve complete denitrification. However, it could achieve complete denitrification when C/N ratio increased to 12, and the correspondin denitrification rate was 0.69 mgN/mgVSS·d.Based on the analysis of denitrification characteristics of different carbon sources, methanol, acetic acid and hydrolysates of food waste were chosen as the external carbon sources for pilot test of biological aerated filter (BAF). The results showed that the dosage of external carbon source to BAF system, TN removal rate increased to more than 75%, and the effluent TN was below 15mg/L. When hydrolysates of food waste were used as the external carbon source, its running cost was much lower than other two carbon sources.