Potential risk and control strategy of biofilm pretreatment process treating raw water

Downflow sponge biofilm reactors for polluted raw water treatment: Performance optimisation, kinetics, and microbial community

Water outages caused by elevated ammonium (NH4+-N) levels are a prevalent problem faced by conventional raw water treatment plants in developing countries. A treatment solution requires a short hydraulic retention time (HRT) to overcome nitrification rate limitation in oligotrophic conditions. In this study, the performance of polluted raw water treatment using a green downflow sponge biofilm (DSB) technology was evaluated. We operated two DSB reactors, DSB-1 and DSB-2 under different NH4+-N concentration ranges (DSB-1: 3.2-5.0 mg L-1; DSB-2: 1.7-2.6 mg L-1) over 360 days and monitored their performance under short HRT (60 min, 30 min, 20 min, and 15 min). The experimental results revealed vertical segregation of organic removal in the upper reactor depths and nitrification in the lower depths. Under the shortest HRT of 15 min, both DSB reactors achieved stable NH4+-N and chemical oxygen demand removal (≥95%) and produced minimal effluent nitrite (NO2--N). DSB system could facilitate complete NH4+-N oxidation to nitrate (NO3--N) without external aeration energy requirement. The 16S rRNA sequencing data revealed that nitrifying bacteria Nitrosomonas and Nitrospira in the reactor were stratified. Putative comammox bacteria with high ammonia affinity was successfully enriched in DSB-2 operating at a lower NH4+-N loading rate, which is advantageous in oligotrophic treatment. This study suggests that a high hydraulic rate DSB system with efficient ammonia removal could incorporate ammonia treatment capability into polluted raw water treatment process and ensure safe water supply in many developing countries.

Emerging wastewater treatment strategy for efficient nitrogen removal and compact footprint by coupling mainstream nitrogen separation with chemical coagulation and biological aerated filter

In this study, an ammonia nitrogen (NH₄⁺-N) ion exchange (IE) and regeneration (AIR) was constructed, and the chemical enhanced primary treatment (CEPT), AIR and biological aerated filter (BAF) were coupled in series to construct a novel CEPT-AIR-BAF process for efficient pollutants removal. At total hydraulic retention time of 4.6 h, the pilot-scale CEPT-AIR-BAF system obtained effluent with chemical oxygen demand of 17.9 ± 6.0 mg/L, NH₄⁺-N of 0.5 ± 0.3 mg/L, total nitrogen of 2.4 ± 1.0 mg/L and total phosphorus of 0.08 ± 0.05 mg/L. AIR module achieved outstanding NH₄⁺-N IE performance with NaClO-NaCl regeneration, and long-term regeneration increased surface area and mesopore of zeolites. Faster-growing heterotrophic bacteria, such as Pseudomonas and Comamonas, were enriched in BAF. The CEPT-AIR-BAF system saved at least 60% of land occupation and upfront investment, and the treatment cost ($ 0.155/m³) should be further reduced by investigations on the regeneration of loaded zeolite.

Biofilm development dynamics and pollutant removal performance of ceramsite made from drinking-water treatment sludge

Alum-sludge ceramsite and denitrifying bacteria (XP-1, XP-2, CL-1, CL-3) were used as substrate and constructed biofilm for enhancing the removal of pollutants from wastewater. The results showed that, due to the large specific surface area, the maximum growth rate was 0.49 mg/(g·day) on the sludge ceramsite, and the mass of biofilm attached onto sludge ceramsite was 5.98 times higher than that when using commercial ceramsite as substrate. Better removal performance could be achieved with the combination of sludge ceramsite and bacteria, viz. 98.6%, 91.0%, and 85.8% reduction in total phosphorus (TP), total nitrogen (TN), and chemical oxygen demand (COD), respectively. Pseudo-first-order kinetics, pseudo-second-order kinetics, Monod kinetics, and multiple Monod kinetics combined with continuous-flow-stirred tank reactor (CFSTR) behavior were used to investigate the dynamics of the pollutant removal processes. The decrease in band brightness for bacteria attached onto sludge ceramsite was 11.5%, while it was more than 35.7% on commercial ceramsite during wastewater treatment according to results from denaturing gradient gel electrophoresis (DGGE). Sludge ceramsite played an important role in maintaining quantities and activities of denitrifying bacteria, and application of sludge ceramsite substrate and denitrifying bacteria was a reliable method to enhance the removals of phosphorus, nitrogen, and COD from domestic wastewater. PRACTITIONER POINTS: Alum-sludge ceramsite was a good substrate for phosphorus adsorption and denitrifying bacterial growth. There was 5.98 times more biofilm on sludge ceramsite than on commercial ceramsite The biofilm of denitrifying bacteria on sludge ceramsite was more stable. High removals of TP (98.6%), TN (90.1%) and COD (85.81%) were achieved.

Calcium ion- and rhamnolipid-mediated deposition of soluble matters on biocarriers

Start-up of biofilm process initiated by the deposition of soluble matters on biocarriers is a very important yet time-consuming procedure. However, rapid start-up methods especially in the enhancement of soluble matters deposition have been rarely addressed. In this study, a quartz crystal microbalance with dissipation monitoring (QCM-D) was applied to investigate the influences of calcium ion and rhamnolipid (RL) on the deposition of soluble matters from real and synthetic industrial wastewaters with different configurations of organics (bovine serum albumin and sodium alginate) and ionic strength on the model biocarriers polystyrene and polyamide. Results showed that deposition was effectively promoted by the addition of Ca²⁺ and along with the increase in Ca²⁺ content. However, RL enhanced the deposition effectively only in hyperhaline wastewater through breaking hydration repulsion and decreased the deposition in low-salinity wastewater, and its influence to the deposited layer property exhibited characteristics of negative feedback. The combined use of Ca²⁺ and RL had a better enhancement effect than that of separate use and the mechanism involved can not be soundly explained only by Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The strategy of mediating the deposition of soluble matters on different biocarriers by adding Ca²⁺ and RL has important implications for regulating biofilm formation to accelerate the start-up process in attached-growth bioreactors.

Optimizing and real-time control of biofilm formation, growth and renewal in denitrifying biofilter

A pilot-scale denitrifying biofilter (DNBF) with a treatment capacity of 600m(3)/d was used to study real-time control of biofilm formation, removal and renewal. The results showed biofilm formation, growth and removal can be well controlled using on-line monitored turbidity. The status of filter layer condition can be well indicated by Turb break points on turbidity profile. There was a very good linear relationship between biofilm growth degree (Xbiof) and filter clogging degree (Cfilter) with R(2) higher than 0.99. Filter layer clogging coefficient (Yc) lower than 0.27 can be used to determine stable filter layer condition. Since variations of turbidity during backwash well fitted normal distribution with R(2) higher than 0.96, biofilm removal during backwash also can be well optimized by turbidity. Although biofilm structure and nirK-coding denitrifying communities using different carbon sources were much more different, DNBF was still successfully and stably optimized and real-time controlled via on-line turbidity.