Treating Urine To Produce Free Nitrous Acids For Sulfide Control In Sewers

Corrosion and odor problem caused by sulfide formation in sewer networks is a global critical challenge to sewer safe operation.According to foreign experience,work of sulfide control should demand huge human and material resources.Currently,the most commonly used methods by dosing chemical agents caused high costs.For that,this study proposed a promising approach by goal-oriented treating urine to produce free nitrous acid（FNA）for sulfide control in sewers.With inoculation of conventional nitrification sludge,the FNA of concentrations could reach up to 4.74 mg HNO₂/L produced in the reactors within 10 days,when known ammonia-oxidizing bacteria（AOB）Nitrosomonas genus with high relative abundance of17.46%became predominance and typical nitrite-oxidizing bacteria Nitrospira genus eliminated.The reactors operated under high and low DO control and maintained over200 days.The results indicated that effect of DO on FNA production is not significant.Insufficient alkalinity of urine is the key factor for FNA accumulation.Therefore,it’s feasible to stably treating urine for high concentration of FNA production.Further,this research set up multiple laboratory-scale sediment reactors for investigating characteristics of sulfide production.The results showed that S^2-、COD、Volatile Fatty Acids（VFA）concentration in sediment was higher than liquid due to the interface transfer limit but SO₄^2-S is the opposite.The methanogenic archaea（MA）mainly included three genus Methansaeta,Methanospirillum and Methanobacterium while sulfate reducing bacterial mainly included two genus Desulfomicrobium and Desulfobulbus.The SRB Desulfobulbus mainly concentrated in the surface zone（0-0.5cm）while the MA Methanospirillum mainly existed in the deeper zone（below 0.5cm）.Besides,the study determined the effect of FNA on sulfide production from sediment.The results showed sulfide concentration decreased from 9.3±1.3 mg-S/L to 0.1±0.1 mg-S/L after 3.43 HNO₂-N/L of FNA exposuring for 24h.The recovering time is 6.3 d when sulfide concentration increased to half of concentation.The measurement of bacterial live/dead ratio at different depth in sediment revealed that the ratio is 89%and 66%,respectively,prior to and after FNA treatment.FNA penetration depth is about 1.2 cm below the seiment surface.Finally,dosing tests in the laboratory-scale sewer systems confirmed that the effectiveness of sulfide control by the urine-generated FNA and life cycle assessment results indicated that producing the FNA from urine of a large commercial building for sulfide control in sewers would save cost of 71%and reduce greenhouse gas（GHG）emissions by 63%compared with dosing chemicals agent in 20years.This research demonstrated perfectly the proposed strategy’s feasibility and advantage,providing technical support and guidance for corrosion and odor control in sewers.