Qiuyan Yuan

VFA generation from waste activated sludge: Effect of temperature and mixing

The success of enhanced biological phosphorus removal (EBPR) depends on the constant availability of volatile fatty acids (VFAs). To reduce costs, waste streams would be a preferred source. Since VFAs were shown to vary in the incoming sewage and fermentate from primary sludge the next available source is waste activated sludge (WAS). The opportunity is particularly good in plants where WAS is stored before shipment. Little information is however available on the rate of VFA release from such sludge, especially at the lower temperatures and under the storage conditions typically found in colder climates. Bench-scale batch tests were performed to investigate the effect of temperature and requirement for mixing on VFA generation from WAS generated in full scale non-EBPR wastewater treatment plant. WAS fermentation was found highly temperature-dependent. Hydrolysis rate constant (k(h)) values of 0.17, 0.08 and 0.04 d⁻¹ at 24.6, 14 and 4°C were obtained, respectively. Arrhenius temperature coefficient was calculated to be 1.07. It took 5 d to complete hydrolysis at 24.6°C, 7 d at 14°C, and 9 d at 4°C. The fermentation lasted for 20 d. At 24.6°C the mixed reactor reached 84% of the overall VFA production only in 5 d. When temperature dropped to 14 and 4°C, the ratio of VFA production at day 10 to overall VFA production in the mixed reactor were 62% and 48%, respectively. The overall VFA-COD concentration in the non-mixed reactors was much lower than the mixed reactors. The information is important for the designer as there was uncertainty with the effect of temperature and mixing on sludge fermentation.

Waste activated sludge fermentation: Effect of solids retention time and biomass concentration

Laboratory scale, room temperature, semi-continuous reactors were set-up to investigate the effect of solids retention time (SRT, equal to HRT hydraulic retention time) and biomass concentration on generation of volatile fatty acids (VFA) from the non-methanogenic fermentation of waste activated sludge (WAS) originating from an enhanced biological phosphorus removal process. It was found that VFA yields increased with SRT. At the longest SRT (10d), improved biomass degradation resulted in the highest soluble to total COD ratio and the highest VFA yield from the influent COD (0.14g VFA-COD/g TCOD). It was also observed that under the same SRT, VFA yields increased when the biomass concentration decreased. At a 10d SRT the VFA yield increased by 46%, when the biomass concentration decreased from 13g/L to 4.8g/L. Relatively high nutrient release was observed during fermentation. The average phosphorus release was 17.3mg PO(4)-P/g TCOD and nitrogen release was 25.8mg NH(4)-N/g TCOD.

Effect of sludge type on the fermentation products.

Primary sludge (PS), waste activated sludge (WAS) and a mixture of WAS and PS were fermented at 21 degrees C. The sludge was collected from two plants: the biological nutrient removal (BNR) West End Water Pollution Control Center (WEWPCC) and carbon-removal only South End WPCC (SEWPCC). The PS fermentation predictably generated a significantly higher amount of soluble COD than WAS. WAS fermentation released considerable amounts of phosphate and ammonium. Co-fermentation of WAS with PS enhanced soluble COD production and increased the release of phosphate and ammonium. The semi-continuous flow fermenters showed that regardless of the sludge source, with a similar total COD load, there was no significant difference in soluble COD production observed during co-fermentation between the two plants. Volatile fatty acids were the primary components of the soluble COD generated during fermentation. 20-22% volatile solids destruction was achieved due to sludge fermentation. The WEWPCC sludge released a higher concentration of phosphate than the SEWPCC sludge - the latter originating from a non-BNR process. Fermentation of combined PS and WAS sludge generated a concentration of phosphate high enough to allow phosphorus recovery as struvite at both plants.

Enhancing biological phosphorus removal with glycerol

An enhanced biological phosphorus removal process (EBPR) was successfully operated in presence of acetate. When glycerol was substituted for acetate in the feed the EBPR process failed. Subsequently waste activated sludge (WAS) from the reactor was removed to an off-line fermenter. The same amount of glycerol was added to the WAS fermenter which led to significant volatile fatty acids (VFA) production. By supplying the system with the VFA-enriched supernatant of the fermentate, biological phosphorus removal was enhanced. It was concluded that, if glycerol was to be used as an external carbon source in EBPR, the effective approach was to ferment glycerol with waste activated sludge.

Biomass fermentation to augment biological phosphorus removal.

A combination of a lab scale biological phosphorus removal sequencing batch reactor (called mother reactor) and a side-stream biomass fermenter was setup. It was found that when fermented biomass was recirculated back into the mother reactor as volatile fatty acid (VFA) supplement, the phosphate concentration in the effluent decreased from 6 in the control reactor to 4.5 mgL(-1) in the effluent from mother reactor. The addition of the fermentation effluent into the mother reactor increased the phosphate and ammonium loads and resulted in deterioration of nitrification. Phosphorus removal and nitrification improved when the fermented biomass was separated from the liquid phase using an up-flow system, followed by the addition of MgO to the supernatant to precipitate phosphate and ammonium. Phosphorus removal was further improved by delaying the time of VFA addition into mother reactor during the anaerobic period as soon as denitrification ceased. Biomass fermentation was found to generate 157 mg VFA-COD by fermenting 1g of biomass at a solids retention time of 5d. Acetate (78% of generated COD) and propionic acid (10%) were the major components of the produced VFA. It was concluded that biomass fermentation to augment a biological nutrient removal process can be effective if generated phosphate and ammonia are removed, e.g. through struvite precipitation.

Influence of carbon source on nutrient removal performance and physical–chemical characteristics of aerobic granular sludge

The impact of carbon source variation on the physical and chemical characteristics of aerobic granular sludge and its biological nutrient (nitrogen and phosphorus) removal performance was investigated. Two identical sequencing batch reactors, R1 and R2, were set up. Granular biomass was cultivated to maturity using acetate-based synthetic wastewater. After mature granules in both reactors with simultaneous chemical oxygen demand (COD), ammonium and phosphorus removal capability were achieved, the feed of R2 was changed to municipal wastewater and R1 was continued on synthetic feed as control. Biological phosphorus removal was completely inhibited in R2 due to lack of readily biodegradable COD; however, the biomass maintained high ammonium and COD removal efficiencies. The disintegration of the granules in R2 occurred during the first two weeks after the change of feed, but it did not have significant impacts on settling properties of the sludge. Re-granulation of the biomass in R2 was then observed within 30 d after granules’ disintegration when the biomass acclimated to the new substrate. The granular biomass in R1 and R2 maintained a Sludge Volume Index close to 60 and 47 mL g−1, respectively, during the experimental period. It was concluded that changing the carbon source from readily biodegradable acetate to the more complex ones present in municipal wastewater did not have significant impacts on aerobic granular sludge characteristics; it particularly did not affect its settling properties. However, sufficient readily biodegradable carbon would have to be provided to maintain simultaneous biological nitrate and phosphorus removal.

Interaction between denitrification and phosphorus removal in a sequencing batch reactor phosphorus removal system.

Batch experiments were performed to investigate simultaneous denitrification and phosphorus removal by denitrifying phosphorus accumulating organisms (DNPAO). The results showed that while using the same amount of carbon source, DNPAOs are able to uptake phosphorus by using nitrate as an electron acceptor. It was observed that higher nitrate concentration required a longer anoxic period for denitrifying phosphate uptake. In addition, the effect of the presence of nitrate in substrate was studied. Phosphorus release occurred as long as substrate was present in the anaerobic phase regardless of nitrate concentrations. It also was noticed that nitrate hindered the release of phosphorus in the anaerobic stage because of denitrifiers competing with phosphorus accumulating organisms for their carbon source. The DNPAOs, however, remained competitive in a system with nitrate in the anaerobic phase.

Polyhydroxybutyrate Production from Municipal Wastewater Activated Sludge with Different Carbon Sources

In this study, a sequencing batch reactor was set up and operated for over three months to cultivate polyphosphate-accumulating organisms polyphosphate-accumulating organisms (PAOs) in the enriched activated sludge. Batch studies were then carried out to study the effect of different carbon sources on phosphorus removal as well as polyhydroxybutyrate (PHB) production. The carbon sources investigated were acetate, glucose, wastewater, and beef extract. It was found that enhanced biological phosphorus removal could not be achieved using glucose as substrate. This suggested that glucose was not a good candidate for biological phosphorus removal. In terms of PHB production, using acetate and glucose as substrate resulted in PHB production of 42% and 40%, respectively, of the dry cell weight (DCW). Lower PHB production was obtained from using municipal wastewater and beef extract as a carbon source. This resulted in ∼15% and 13% of DCW. It was concluded that municipal wastewater activated sludge can be an economic alternative for PHB production if municipal wastewater is mixed with certain kinds of carbon-enriched industrial wastewater.

Mechanism of nitrogen removal in wastewater lagoon: a case study

ABSTRACT Ammonia being a nutrient facilitates the growth of algae in wastewater and causes eutrophication. Nitrate poses health risk if it is present in drinking water. Hence, nitrogen removal from wastewater is required. Lagoon wastewater treatment systems have become common in Canada these days. The study was conducted to understand the nitrogen removal mechanisms from the existing wastewater treatment lagoon system in the town of Lorette, Manitoba. The lagoon system consists of two primary aerated cells and two secondary unaerated cells. Surface samples were collected periodically from lagoon cells and analysed from 5 May 2015 to 9 November 2015. The windward and leeward sides of the ponds were sampled and the results were averaged. It was found that the free ammonia volatilization to the atmosphere is responsible for most of the ammonia removal. Ammonia and nitrate assimilation into biomass and biological growth in the cells appears to be the other mechanisms of nitrogen removal over the monitoring period. Factors affecting the nitrogen removal efficiency were found to be pH, temperature and hydraulic residence time. Also, the ammonia concentration in the effluent from the wastewater treatment lagoon was compared with the regulatory standard.

Seasonal variations in cold climate nutrient removal: A comparison of facultative and aerated lagoons

The seasonal trends in standard wastewater parameters are studied for two lagoons in the Canadian Prairies; one facultative and one aerated with the purpose of better understanding the underlying biological mechanisms in place. In particular, treatment in a cold climate is examined as treatment efficiency and function vary with geographical latitude. It was found that during the winter season, nutrients are not removed and nutrient release is observed. At the arrival of spring, biological growth occurs leading to spring awakening of the lagoons whereby nutrients start to again be removed. Phosphorus is removed by biomass assimilation and precipitation. It was found that these mechanisms were not very effective at treating this nutrient and additional treatment is required. Nitrogen is removed mainly by air stripping and its concentration is influenced by both temperature and pH, the latter of which is greatly affected by algae growth.