Impact Of Fine Sediment On Activated Sludge Characteristics

As the lag of drainage facilities construction, influent sediment concentration ofwastewater treatment plant was relatively high. A great amount of fine sediment(200μm) enters into the subsequent biological treatment system, because of the poorremoval efficiency of fine sediment in grit chamber and the cancellation or by-pass ofprimary settling tank, causes many problems, such as deposition of fine sediment inbio-chemical unit, MLVSS/MLSS decreases, low organic content in excess sludge andso on, which will impact the normal operation of biological wastewater treatmentsystems. On the condition of low MLVSS/MLSS ratio, wastewater treatment plantshave to elevate the sludge concentration to maintain enough activated microorganism,and to ensure the system keeping good running state. On the basis of review of theorigin of fine sediment in wastewater, the sediment carrying by runoff during the rainyseason is considered as the main source of influent fine sediment in WWTPs,.Obviously, the fine sediment in the mixed liquor of bio-chemical will decrease with thedischarge of excess sludge from the system, resulting the increase of MLVSS/MLSS ofactivated sludge, when the water quality returns to normal after rainy season. In orderto achieve low-power operation of WWTPs and guarantee the treatment effect, it’simportant to control the sludge concentration (MLSS) reasonably, according to thechange of MLVSS/MLSS ratio. However, there is no related research has been found.Based on the above discussion, focusing on decease of MLVSS/MLSS and depositionof fine sediment, and aiming at providing scientific theory basis for operation ofWWTPs, the impact of fine sediment shocking load (measured in ISS/COD，unit as mgISS/mg COD) on the MLVSS/MLSS and other activated sludge characteristics werestudied by artificially adding fine sediment into the influent to simulated water qualityduring one-time storm period. In addition, impact of fine sediment load during rainyseason on the MLVSS/MLSS, other activated sludge characteristics, the recoveryperformance of MLVSS/MLSS and other activated sludge characteristics were alsostudied. Finally, a new predictive model for the fine sediment concentration in thebio-chemical unit is presented, and it’s beneficial to dynamic control of sludgeconcentration. The main conclusions are as follows:①Fine sediment has no significant impact on microbial activity (SOUR, measured in MLVSS), but it will reduce MLVSS/MLSS ratio, resulting decrease of activity per unit mass of sludge (SOUR1, measured in MLSS). It’s found that MLVSS/MLSS ratio reduces with the increase of fine sediment load. At shocking load of fine sediment (4.8≥ISS/COD≥1.2), MLVSS/MLSS reduced from0.803-0.821to0.704～0.735, about10.4%±1.6%in average. For further study, fine sediment load is the main influence factor of MLVSS/MLSS ratio, and the higher the load, the greater the degree of influence. The minimum MLVSS/MLSS has good linear relationship with ISS/COD (MLVSS/MLSS=-0.2165r-+0.7611, Radj2=0.9637, r refers to ISS/COD) at impact of fine sediment. WWTPs should real time control the sludge concentration basing on the MLVSS/MLSS ratio to maintain a sufficient amount of active microorganisms (MLVSS) ensuring the system at good running state.②The increase proportion of fine sediment in activated sludge results in better dewatering performance and settling performance. At shocking load of fine sediment, sludge settling and dewatering performance turned better shortly. Further study shows that the sludge settling and dewatering properties turn better with the increase of fine sediment load and the influence time, and the minimum SVI has good linear relationship with ISS/COD (SVI=11.6r2-54.6r+94.7, Radj2=0.9467) at impact of fine sediment. In addition, average CST during influence period has good linear relations with ISS/COD (CST=-0.2685r+1.9247, Radj2=0.7865).③Fine sediment can be considered as a carrier on which biomass grows forming a thin bio-film, and it’s beneficial to forming stable sludge floc with large particle size at high fine sediment load. It’s found that sludge particle size increases with ISS/COD≥2.4mg ISS/mg COD, and average sludge particle size (Dv) has good linear relationship with ISS/COD (Dv=6.49r+133.6, R2adj=0.8022) at shocking load of fine sediment. Further study shows that sludge particle size during influence period and ISS/COD also has good fitting relationship (Dv=142.30-25.30r+10.60r2, R2adj=0.8852) at impact of fine sediment.④From the point of MLVSS/MLSS, it will take longer time for the system to recover from the influence of higher fine sediment load. At shocking load of fine sediment, the recovery time was about7days with ISS/COD less than2.4mg ISS/mg COD, while the recovery time was about19days with ISS/COD more than3.6mg ISS/mg COD. When systems run to steady state at constant influent fine sediment load, recovery time was about44days for the system with ISS/COD of0.6mg ISS/mg COD, while the recovery time of systems with ISS/COD more than1.2mg ISS/mg COD were more than56days.⑤Fine sediment migration model shows that the concentration of fine sedimentsuspended in mixed liquor (XISS) is proportional to influent concentration of finesediment (ISS), proportion of suspending fine sediment (θ), sludge age (ts), influencetime, and inversely proportional to hydraulic retention time (th) during influence period,while it is proportional to original concentration in reactor, sludge age (ts), andinversely proportional to influence time during recovery period. Validity analysisbased on the actually measured value of XISSdemonstrates that the actual values fitwell with the calculated values of influence and recovery period, with relative error of11.7%-24.6%and29.1%-46.0%, respectively.⑥The deposition rate (w) increases with the fine sediment load, and w has goodrelationship with ISS/COD ratio (w=8.260r-0.2065,R2adj=0.9982). The particle sizedistribution of the deposition is larger than that of the fine sediment in the influent withmean volume average particle diameter of85.24±5.87μm, indicating that finesediment with larger particle size is easier to deposit at the bottle of reactor. In addition,the deposition of fine sediment will decrease the effective volume of the reactor.