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Description

The desire to recover useful resources, especially nutrients from waste streams, is continuously increasing. Leachate and digester reject water or centrate are two of the most challenging urban waste streams that pose enormous challenges in handling. Leachate is a result of rainwater seeping through a landfill and mixing with contaminants produced by biological and chemical processes within the landfill, as well as water squeezing out from the solid waste. Leachate contains organic matter, ammonia, heavy metals, organic and inorganic salts (Renou et al., 2008). Anaerobic digester reject wash water and belt press black liquid together known as centrate or filtrate is another form of urban waste stream which is small in volume but very concentrated in contaminants, primarily ammonia nitrogen and phosphorus. Centrate is generated when the anaerobically digested sludge in wastewater treatment plants is dewatered, mostly using centrifuges or belt presses. In most of the wastewater management facilities, centrate is returned to the head of the plant, and as a result, the facility encounters excess operation and maintenance costs such as those associated with chemical addition and energy consumption (Kotay et al., 2013). Introducing centrate to the influent wastewater increases the nitrogen and phosphorus load which may end up in the effluent (Holloway et al., 2007). Treating centrate separately could reduce associated costs and improve the quality of the effluent (Holloway et al., 2007). The most common side stream treatment technologies for wastewaters are, membrane bioreactors (MBR), sequencing batch reactors (SBR) and configurations incorporating sequencing batch reactor and anaerobic side-stream reactors (Cannibal®, BIMINEX®), anaerobic ammonium oxidation (anammox) and many others.

In this study, a new treatment strategy, formulated around the theme of co-management of leachate and centrate, was developed to recover useful resources and treat the residues. Struvite precipitation process to recover/remove phosphorous (P) and nitrogen (N) was optimized and employed. Aerobic granular sludge (AGS) process to simultaneously remove carbon, nitrogen, and single-stage partial nitritation/anammox (PN/A) process to remove residual nitrogen were used as biological treatment processes.In the integrated management scheme, the influent was a mixture of anaerobic digester centrate and real leachate in a 4:1 ratio. Almost 77% recovery of phosphorus and 25% removal of NH4+-N were accomplished through struvite precipitation at an optimum pH of 9. High pH contributed to free ammonia loss during struvite precipitation experiments. In the aerobic granular sludge reactor overall, BOD5, COD, and NH4+-N removal percentages were 74%, 45% and 35%, and in the PN/A reactor, overall 35% removal of total inorganic nitrogen (TIN) was observed. More than 80% BOD removal was recorded in the granular reactor with soluble COD (sCOD) removal fluctuating between 28% and 57%, depending on the operational phase. Exposing the synthetically cultured aerobic granules directly to the mixture of leachate and centrate unveiled an alteration in the physical characteristics of granules; however, reactor operational data and microbial community analysis ascertain the effectiveness of the treatment scheme treating two urban waste-streams.

Nitrogen management for mature landfill leachate with low readily biodegradable organic content, but high ammonia-nitrogen content was also evaluated using single-stage attached growth anammox process. Nitrogen removal rates were studied from the perspective of the potential nitrogen conversion/removal mechanisms incorporating different components from the overall nitrogen cycle. In the presence of organic carbon in leachate, the occurrence of denitrification, its effect on overall nitrogen removal, and corresponding inhibition on anammox were investigated.

Simultaneous functional gene expressions using mRNA, rate measurements, and biochemical analysis using heme c protein (unique marker in the anammox pathway), proved the consistent contribution of ammonia oxidizers, heterotrophic denitrifiers, and anammox bacteria in total inorganic nitrogen removal from mature landfill leachate. The presence of heme c protein and expression of hydrazine synthase (hzsA) genes, explicitly confirmed the activity of anammox microorganisms in the reactor. Average removal efficiencies of ammonia–nitrogen, total inorganic nitrogen, and COD were 94%, 88%, and 26%, respectively, in the reactor. Off-gas N2O fluxes increased at relatively higher dissolved oxygen concentrations. Batch activity tests revealed the occurrence of significant anammox activity even in the presence of high concentrations of organic carbon in the influent. mRNA based functional expressions of nitrite reductase (nirK and nirS) and hydrazine synthase (hzsA) suggested simultaneous active heterotrophic denitrification and anammox, respectively.

In order to assess the microbial community developed in each of the biological reactor in each process, and to determine the shifts in microbial community with the change in reactor operating phases, high-throughput amplicon sequencing of the 16S rRNA gene, targeting the V4 hypervariable region was conducted. Analysis was carried out to determine the taxonomic classification, which can be used to predict relative abundances of target microbial communities. For the SGA process, Proteobacteria, Bacteroidetes, Nitrosomonas were observed as the most dominant phyla. The presence of the following family, Rhodobacteraceae, Xanthomonadaceae, Flavobacteriaceae, in the granular biomass, confirmed the defined redox zones inside mature granules indicating simultaneous removal of nitrogen (N) and organics in the aerobic granular sludge technology. For the PN/A reactor, Planctomycetes was observed as the most dominant phylum.

A cost analysis study was carried out in order to assess the environmental impact of the proposed novel treatment approach and to compare it to more traditional landfill leachate treatment approaches such as MBR, SBR, and direct discharge through publicly owned treatment works (POTW). The study was completed with the use of spreadsheet-based models and literature review. Spreadsheets have been developed to evaluate treatment costs associated with capital, operations, and maintenance for the proposed nutrient-recovery process and traditional on-site leachate treatments. Transportation costs for leachate to the WWTP have been analyzed as a function of distance. Results suggest that treatment using SGA is lower in cost and comparable in efficiency to the traditional approaches SBR, and MBR. Positive outcomes also include lower power, labor, and chemical cost, lower CO2 and N2O emissions, recovery of nitrogen and phosphorus, and production of struvite fertilizer.

The engineering and microbial data, with corresponding cost analysis data, that were discussed in this study, provide complete guidelines in full-scale installations of the treatment train in the management of two concentrated urban waste-streams such as leachate and centrate, making the study novel and outstanding. The results from this study will also provide guidelines for the mainstream application of a single-stage anammox process in landfill leachate as well as municipal wastewater treatment plants. This research showed for the first time that indeed nutrient recovery could be coupled with simultaneous carbon and nitrogen management in landfill leachate, which can be beneficial in overall solid waste management.

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