Treatment and Reuse of Black Water by Novel Energy-saving Shaft/Anaerobic /Anoxic/ Aerobic (S/A/A/O) System: A Novel Energy-saving Shaft/Anaerobic /Anoxic/ Aerobic (S/A/A/O) System for black water

In this study the novel integrated anaerobic/anoxic/aerobic (A/A/O) and shaft aeration process system which could enhance the nutrient removal efficiency of wastewater with a low carbon nitrogen ratio (C/N) was investigated. Several dissolved oxygen (DO) ratios (1.0-2.0, 2.0-3.0, 3.0-4.0 mg/L) were applied in order to obtain more knowledge on the biological nitrogen and phosphorus removal performances. The experiment was carried out on a lab-scale Shaft-Anaerobic-Anoxic-Aerobic (S/A/A/O) wastewater treatment system. The average removal efficiencies of the total organic carbon, total nitrogen and phosphorus were 84.0%, 82.0% and 93.7%, respectively. The result showed that the proposed system was promising for the treatment of wastewater with a low C/N ratio because that in this system nitrification and denitrification happened simultaneously. Besides, the system had an automatic return system and its floor area was small, which made it energy-saving.


Introduction
It is widely acknowledged that the biological nutrient removal (BNR) system was suitable for the wastewater treatment plants nowadays because it can remove nitrogen and phosphorus simultaneously and has an economic advantage and a good flexibility [1].
The Anaerobic/Anoxic/Aerobic (A/A/O) process has been famous for its ability to remove nitrogen and phosphorus at the same time. However, this process still has some main operational problems [2,3]. Firstly, the circulation pumps are not very energy efficient.
What's more, traditional composting systems cover large areas and diffuse foul smell. Secondly, the system has some conflicts between the sludge retention time (SRT) and polyphosphate accumulating organisms (PAOs). Thirdly, in the anaerobic area the denitrifiers and PAOs compete for the external carbon source, while the phosphorus released is inhibited by NO 3 − -N in the return sludge. Finally, the nitrogen and phosphorus removal rate is limited by the low C/N ratio of influent [3,4].
However, black water (or toilet wastewater) is generally low in the C/N ratio and the discharge of one ton black water can pollute 220 tons clean water [5]. Nevertheless, the black water often lacks proper treatment before going to the wastewater treatment plants, which may cause serious problems.
Therefore, it is of great importance to conduct research on technologies for black water treatment with higher efficiencies and lower costs [6]. The aerobic treatment of black water for recycling from toilet flushing is a promising alternative method.
Membrane bioreactor (MBR) is one of the technologies to treat toilet wastewater, but this process has a high-energy consumption. Besides, cleaning and maintenance of the membrane is rather complicated [7,8].
Therefore, it is a challenge to obtain sludge operation process with lower energy consumption and operation cost to meet the strict discharge standard. In order to solve the problems of traditional methods mentioned above, a new energy-saving for black water treatment and recycling was proposed.  [9,10]. Thirdly, instead of the anaerobic zone NO 3 -N is transferred to the anoxic area to guarantee the anaerobic environment for phosphate release [11], which also avoided the competition between denitrifiers and PAOs for external carbon source in the anaerobic zone. Finally, large amount of TOC in black water with a low C/N ratio is used in the anaerobic zone of S/A/A/O reactor, which is beneficial for the nitrification and then stimulate the growth of denitrifying polyphosphate accumulating organisms (DPAOs) which are capable to use excess NO 3 − -N as electron acceptors. Therefore, the proposed process is promising and has a lot of advantages including removing nitrogen and phosphorus at the same time [12]. In this research, the automatic return systems of nitrate and sludge were investigated due to its good economical advantage in S/A/A/O system. The effects of DO which is relative to the nitrate recycling ratio on the nitrogen and phosphorus removal effecincy of the system was observed by treating real black water.
Simultaneously, the energy consumption as well as the operational cost of the S/A/A/O process was analyzed.
To the best of our knowledge, this study is the first academic report about treatment and reuse of black water with S/A/A/O process.

Wastewater source
The wastewater used was taken from the residential area of Guilin University of Technology in Guilin, China. The main characteristics of the influent were summarized in Table 1.
Where N SND , N denitrification and N assimilation are, the nitrogen removed by SND process, denitrification process and biomass assimilation process, g N/d. N influent and N effluent are the nitrogen in the influent and effluent, gN/d, respectively [13].

Calculation of anoxic P balance concentration in S/A/A/O process
Based on material balance, anoxic P balance concentration (U, mg/L) could be calculated according to Eq. 3: Where R and r is the sludge and nitrate recycling ratio, respectively; P a is the PO 4 3− -P concentration in the effluent of the last anaerobic zone; P b is the PO 4 3− -P concentration in the effluent of the last aerobic zone [14].

Operational conditions
The operational conditions which were chosen in the whole experimental period were presented in Table 2.     microbial communities in the reactor. In the anaerobic environment a lot of TOC was consumed. Since the TOC had a non-inhibitory effect, this consumption was good for nitrification which resulted in the enhancement of TN removal [13]. Therefore, the optimal DO concentration for maximum efficiency of SND had a significant difference in this process. When treating the black water with a low C/N ratio, it is often effective to increase phosphorus removal ratio in the S/A/A/O process which often increase the N and P removal rate significantly. In the denitrifying phosphorus removal process, the same carbon source can be used for both N and P removal. Besides, the nitrate concentration in the anoxic reactor determined whether the DPAOs were active [19]. The denitrifying phosphorus removal was affected by the nitrate recycling ratio. It also explained the result that TP removal efficiency in anoxic zone had a bit increase when increasing the nitrate recycling ratio in Fig. 2.

Variation of TOC in the influent and effluent
and their removal efficiencies

Conclusions
The primary findings of this study are as below: (1) The S/A/A/O process with automatic return systems could enhance the nutrient removal performance in treatment of low C/N ratio wastewater. (2) The average effluent concentrations of TOC, TN, NH 4 + -N and TP were 18.5, 14.4, 5.8 and 0.8 mg/L, respectively.
(3) The DO which influenced the nitrate recycling ratio is an important operating parameter.