This work reports a new waste activated sludge (WAS) pretreatment method based on free ammonia (FA) for promoting the generation of short-chain fatty acids (SCFAs). Experimental results showed that pretreatment of WAS for 3 days with FA largely improved WAS disintegration, with the highest dissolution (soluble chemical oxygen demand (COD), 3400 ± 120 mg/L at initial FA level of 237.8 mg/L) being 4.5-fold that without FA pretreatment. The pretreatment method by FA facilitated the breakdown of extracellular polymeric substances and cell envelope of sludge cells and killed more live microbial cells, which thereby accelerated the dissolution of substances from WAS. It was also found that FA severely suppressed the SCFA consumption process, but the acetogenesis process was unaffected. Although FA also inhibited hydrolysis, acidogenesis, and homoacetogenesis to some extent, the inhibitions did not largely affect the biodegradation of the relevant substances at all the tested FA levels. Finally, using FA to pretreat WAS for SCFA enhancement was confirmed. When FA concentrations ranged from 53.5 to 176.5 mg/L, the maximum generation of SCFA was enhanced from 196.8 to 267.2 mg COD/g VSS, which was 2.3−3.2 times that of the blank. Further FA leveling (237.8 mg/L) caused a slight decline of maximum SCFA generation (226.9 mg COD/g VSS). The findings reported may instruct engineers to develop an economic and effective strategy to enhance SCFA production, which might support the operation of wastewater treatment plants in sustainable paradigms with low energy input in the future.
With the implementation of One Belt And One Road plan in 2013, Chinese enterprises have gradually increased their investment in transnational PPP infrastructure projects. Due to the difficulty of transnational investment and asymmetric information, the engineering risk factors of transnational PPP infrastructure project investment deserve everyone’s attention. Based on the literature review, this paper identifies the engineering risk factors of current transnational PPP infrastructure projects. Meanwhile, according to the experience of domestic PPP projects allocation and the experience of enterprise interviews, it comes to the engineering risk management focus of Chinese enterprises when they invest in transnational PPP infrastructure projects.
As is well-known, risk allocation played a key role in the process of risk management of PPP municipal infrastructure projects. In China, thousands of PPP projects are initiated by all levels of local governments. However, it is not known whether the risk allocation plan came true in contract. How to identify risk factors, methods of risk allocation and risk response measures were payed attention by current researchers in PPP risk area. But there were few students focusing the question about contract realization of risk allocation plan, especially in Chinese municipal infrastructure projects. Based on case study method, this paper proposed that most of the risks which allocated in execute plan are actually implemented in the contract, but not all. This research can militate positive impacts for the risk allocation and contract signing process, so as to enhance the effect of risk allocation and improve the value for money of Chinese PPP municipal infrastructure projects.
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