Recovering nutrients from wastewater treatment plants through struvite crystallization: CFD modelling of the hydrodynamics of UBC MAP fluidized-bed crystallizer

This paper is an attempt to model the hydrodynamics of the UBC (University of British Columbia) MAP (magnesium ammonium phosphate) fluidized bed crystallizer. In this study, a numerical investigation of hydrodynamics of the UBC MAP crystallizer was performed, using commercial computational fluid dynamics (CFD) software, Fluent 6.3. One of the main findings of this modelling effort is the relative distribution of the different phases, in this case solids (struvite crystals) and liquid (water), within the reactor. This information is very important, in the sense that it helps fixing the size of the crystallizer, which can be a significant factor, affecting the total cost of the process. Another finding of this simulation is the volume fraction distribution of the different sizes of solid particles, which actually provides insight into the hydrodynamics of the reactor and will fill up the knowledge gap in developing an overall mechanistic model for the crystallizer.</jats:p

The effects of macromolecular substrates and a metabolic inhibitor on volatile fatty acid metabolism in thermophilic aerobic digestion

The effects of spiking macromolecular substrates and an inhibitor on the metabolic behavioral patterns of thermophilic aerobic digestion (TAD) biomass, from a pilot scale system, were studied. The 3 macromolecules examined were linoleic acid (lipid), peptone (protein) and dextrin (carbohydrates). Linoleic acid did very little in terms of volatile fatty acid (VFA) stimulation in either anaerobic or microaerobic environments. Dextrin stimulated propionate and acetate production in a rough 2 propionate to 1 acetate molar ratio under anaerobic conditions. Peptone was the only one of the three tested substrates that stimulated only acetate production under fermentative conditions. Under microaerobic conditions (oxygen demand exceeds oxygen supply), both peptone and dextrin stimulated the production of acetate. The results seen under both anaerobic and microaerobic experimental conditions, with the exception of linoleic acid, in so far as redox balance is concerned, are consistent with the hypothesized biochemical mechanisms describing substrate metabolism in TAD proposed by Chu et al. (1996). Since fermentative reactions do not utilize oxidative phosphorylation to produce energy, 2,4-dinitrophenol had no measurable effect on VFA metabolism in an anaerobic environment. The effect of this agent under microaerobic conditions, was to induce the process biomass to switch from generating energy via oxidative means to using substrate level phosphorylation reactions. This results in the accumulation of large amounts of acetate in excess of the control condition that was not treated with 2,4-dinitrophenol, since acetate production from acetyl-Coenzyme A yields energy.</jats:p

Biological nitrification and denitrification of a simulated high ammonia landfill leachate using 4-stage Bardenpho systems: system startup and acclimation

This research investigated the nitrogen removal capability of two biological nitrification systems, with pre- and post-denitrification, when treating a landfill leachate characterized by high ammonia concentrations and low levels of biodegradable organics. The recycle ratios of the systems were set so that, at an average influent flow of 10 L/d, the actual hydraulic retention time of the first anoxic reactor was about 1.5 h for one system and 1.7 h for the other system. The systems also operated at a first aerobic reactor actual hydraulic retention time of 3 and 3.4 h, respectively. Methanol was used as a supplementary organic carbon source for denitrification. High leachate ammonia concentrations were simulated by artificially increasing influent ammonia to about 2200 mg N/L. This paper presents an overview of initial startup and acclimation, as well as some of the direct and indirect effects of methanol addition on process performance. The reported data were collected during two runs at incrementally increasing influent ammonia concentrations. During the first run to reach 2200 mg N/L, methanol loading rates were increased concomitantly with ammonia loading rates, to match expected aerobic NOx production, using a CH3OH:NOx of about 20:1. This resulted in methanol carry-over into the first aerobic zone, enhanced aerobic heterotrophic growth, and further inhibition of the nitrifying population, already inhibited by recycling through the elevated "free" ammonia levels of the first anoxic zone. When these systems were allowed to adapt up to 14 days, rather than 7 days, initially, to each incremental ammonia increase, and with methanol loading rates subsequently changed to yield CH3OH:NOx of only 5:1, the influent ammonia concentration was increased to approximately 2200 mg N/L within 88 days from the start of the second run, without any inhibitory problems. The timing and levels of ammonia and methanol loading rate increases, with respect to each other and to the corresponding previous loading rate increase, played an important role in system stability and the onset of nitrification failure.Key words: biological treatment, high ammonia leachate treatment, denitrification, methanol, nitrification. </jats:p

Improvement in aerobic sludge digestion through pH control: initial assessment of pilot-scale studies

Waste-activated sludges from an extended-aeration, pilot-scale wastewater treatment facility and a high-rate, full-scale system were aerobically digested in 150 L pilot-scale digesters, operated in a semicontinuous (daily fill-and-draw) mode, at a standard 15-day solids retention time (SRT). To offset the mixed-liquor pH (MLpH) decrease normally encountered in these digesters (as a result of nitrification), hydrated lime and sodium bicarbonate were used in separate experiments to control MLpH in the series pH 6, 7, 8, and 9. Digester performance in the first stage of this work was assessed solely on the basis of reduction in total volatile suspended solids.The extended-aeration type sludge exhibited the greatest improvement in process performance under all pH-controlled conditions. Improvements in digestion efficiency of up to 80% over the uncontrolled reactors were noted. The use of lime resulted in greater digestion enhancement than did sodium bicarbonate with this sludge, without a significant increase in sludge solids production (owing to the low chemical doses required). Digestion efficiency of the high-rate type sludge was little improved (on a relative basis) with either chemical; however, absolute efficiencies in the individual digesters were, in some cases, nearly double those of the comparative extended-aeration sludge digesters. This difference appears to be a function of the process from which the digesting sludge originated, and seems to be influenced by the amount of easily oxidizable, endogenous substrate contained in the biomass. It was concluded that the extended-aeration type sludge was most amenable to enhanced digestion through pH control; as well, cost and process considerations made lime the chemical of choice. Key words: activated sludge, aerobic digestion, hydrated lime, mixed-liquor pH control, nitrification, process enhancement, pilot-scale, sodium bicarbonate. </jats:p

Wastewater biosolids: an overview of processing, treatment, and management

Treated as a valuable resource, municipal sludge, often today referred to as biosolids, is processed through a variety of novel unit operations leading to a safe, aesthetically pleasing, and sought-after product. The design engineer is concerned first with the ultimate disposal and utilization of the biosolids, providing at least two options for the final disposal. Volume reduction, stabilization or vector attraction reduction, and pathogen inactivation are the key goals; process trains combining them into one unit process are the target technologies. Drying and pelletization are now being applied at much smaller plants because of the introduction of indirect dryers, which have fewer air pollution problems than the direct dryers still used at some larger plants. Stabilization of biosolids in newer plants is more often combined with disinfection at thermophillic temperatures, in anaerobic and particularly in aerobic regimes. For the smallest plants, dewatering is now available in drying bags or vacuum drying beds, and larger plants benefit from an array of new devices offering sludge cakes as dry as 22 to [Formula: see text]40% total solids. The ultimate dryness will depend on the quality of sludge, polymer conditioning program, and machine parameters. Emphasis on cost reduction, with simultaneous demand for an excellent quality end product, calls for innovative and case-specific solutions that go beyond the treatment plant and also address the quality of industrial-commercial discharges to the municipal sewers.Key words: sludge, biosolids, process design, dewatering, digestion. </jats:p

Pre-denitrification and pre- and post-denitrification treatment of high-ammonia landfill leachate

This bench-scale study investigated the nitrogen-removal capabilities of two different biological process configurations treating methanogenic-state landfill leachate containing up to 1200 mg N/L of ammonia. The first configuration was a pre-denitrification system known as the modified Ludzack-Ettinger (MLE) process. Large clarifier sludge recycle flows, set to yield clarifier recycle ratios of 7:1 and 8:1, were evaluated as a means to reduce effluent NOx concentrations. A pre- and post-denitrification system, known as the four-stage Bardenpho process, was the second configuration evaluated. The MLE systems (20 day aerobic solids retention time (SRT)) were capable of producing effluent containing about 50 mg N/L of ammonia and 200-235 mg N/L of total inorganic nitrogen (ammonia + NOx) when treating leachate containing approximately 1200 mg N/L of ammonia. In contrast, effluent from the four-stage Bardenpho system contained less than 1 mg N/L of ammonia and 15 mg N/L of NOx, when treating 1100 mg N/L ammonia leachate. An aerobic number 1 SRT of 20 days (total aerobic SRT approximately equal to 40 days) was used with aerobic number 1 and clarifier sludge recycle ratios of 4:1 and 3:1, respectively. The ammonia-removal potential of both systems was clearly demonstrated but each system also showed certain disadvantages, characteristic of each process.Key words: ammonia-N, anoxic denitrification, leachate treatment, nitrification, pre-denitrification. </jats:p

Aerobic sludge digestion at cold temperatures

Continuous flow, daily fill and draw, and batch aeration digesters were studied on a laboratory scale, to develop low temperature characteristics and design criteria for aerobic digestion of waste-activated sludge. These results were compared against full-scale data, from three independent sources. Raw sludge used in these studies was obtained from a municipal high-rate activated sludge plant. The digestion systems were operated at temperatures of 20, 10, and 5 °C, and at six different sludge ages. Measurements of all parameters studied were made under steady-state conditions.The results show that the effect of low temperature on aerobic digester performance is pronounced. The combined effect of temperature and sludge age is shown to be an important design parameter. Kinetic reaction rates and temperature coefficients were calculated on the basis of total volatile suspended solids. It was shown that batch and continuous feed system reaction rates are not interchangeable. The laboratory results further show that there is little difference between continuous flow and daily fill and draw digestion characteristics, except at 5 °C. Difficulties encountered in making a valid comparison between the laboratory and full-scale results are also discussed. </jats:p

Preliminary assessment of a shortcut in nitrogen removal from wastewater

The objective of this long-term research project was to demonstrate the feasibility of removing nitrogen from highly nitrogenous wastewater by (a) blocking the nitrification process at the intermediary nitrite level through the action of free ammonia and (b) subsequently reducing the nitrite to nitrogen gas. The success of such a process could lead to substantial reductions in nitrogen removal costs.Two identical bench-scale activated sludge systems were operated for 147 days, in the initial phase. Each system was composed of four equal-sized, completely mixed cells in series. The free ammonia concentration was highest in the first cell of each system. It averaged 2 mg NH3-N/L in the first system and 5 mg NH3-N/L in the second. Nitrite buildup, in excess of 80% of the oxidized nitrogen present, was induced and sustained for around 2 months in all cells of the second system, after which time a steady decline occurred. Nitrite buildup could not be sustained in the first system. Average chemical oxygen demand (COD) for nitrite reduction was 40% lower than that for nitrate reduction. The nitrification rate for the ammonia oxidizers was similar for both systems. The presence of up to 100 mg [Formula: see text] nitrite in system 2 caused no discernible inhibition. Subsequent runs proved that nitrite accumulation could not be sustained indefinitely, owing to acclimation to free ammonia levels as high as 22 mg NH3-N/L. Periodic resting and flushing may be required; further research is being pursued along these lines. Key words: biological treatment, denitrification, nitrification, nitrite, nitrogen removal, nitrogenous wastewater. </jats:p