Pre-denitrification Process Research Articles

Water-Energy Nexus in the Antofagasta Mining District: Options for Municipal Wastewater Reuse from a Nearly Energy-Neutral WWTP

The region of Antofagasta is the mining hearth of Chile. The water requirement of the local mining sector is 65% of the total water uses, with a water consumption of approx. 9 m3/s in the year 2020. That determines an important pressure onto freshwater, which can only be alleviated by resorting to desalination or reuse of treated wastewater. At present, an amount equal to 90% of the wastewater generated in the city of Antofagasta is discharged into the ocean, after undergoing only preliminary treatments. The wastewater treatment plant (WWTP), which includes a conventional activated sludge (CAS) process, has a very low treatment capacity, insufficient to serve the whole population. A new WWTP will be built with the twofold aim of (i) purifying the totality of the wastewater generated from the city (approx. 320,000 equivalent inhabitants, e.i.), and (ii) allowing the reuse of 100% of the treated wastewater in the local mining sector, in agreement with the goals of the Chilean government. The new Antofagasta WWTP will include preliminary treatments and a conventional activated sludge (CAS) process with a higher treatment capacity. This study integrates a number of modeling tools, namely the Activated Sludge Model n.3 (ASM3), the Takacs model, and some stoichiometric and energy balances, to assess the impact that some changes, possibly introduced into the project of the new WWTP, could determine on its energy and environmental sustainability. Specifically, through an energy-economic-environmental (3-E) analysis, the original scheme of the planned WWTP was compared with three scenarios, of which Scenario 1 introduces anaerobic digestion (AD) of secondary sludge, Scenario 2 concerns primary sedimentation and AD of both primary and secondary sludge, and, finally, Scenario 3, other than primary sedimentation and AD, also includes a pre-denitrification process. The results of the study demonstrated that all the changes introduced by Scenario 3 were of capital importance to promote the transformation of the WWTP into a nearly energy-neutral water resource recovery facility (WRRF). Specifically, the processes/operations introduced with Scenario 3 can reduce the electric energy demand from external sources to only 20% of that of the original scheme, and consequently avoid the emission of 4390 tons CO2-equivalent/y.

Towards energy neutrality by optimising the activated sludge process of the WWTP Bochum-Ölbachtal.

As a result of Ruhrverband's regularly performed energy audits the wastewater treatment plant (WWTP) Bochum-Ölbachtal shows substantial deficits concerning energy efficiency. Due to the energy consumption of internal recirculation, mixers and return activated sludge (RAS) pumping the existing pre-denitrification process configuration offers a specific energy consumption for biological treatment of 23 kWh (PE · a)(-1). In order to optimise the energy situation and to improve the treatment efficiency, the process layout was changed completely to a three-stage step-feed process. By optimising the hydraulic conditions, it was possible to reconstruct the plant with a free flow throughout the whole biological treatment system without any additional pumping. The total investment costs for this process scheme were 3.9 million €. These costs could be partly offset against the wastewater charge paid (2.9 million €). Compared to the overall energy consumption before the process modification, today the energy consumption for biological treatment amounts to 12.4 kWh (PE · a)(-1). The highest saving potential has been achieved by optimising mixing and reducing the energy demand for internal recirculation and RAS pumping. In the case of the WWTP Bochum-Ölbachtal, the modification of the treatment process not only results in an improved energy situation but also increased the treatment efficiency in such a way that the nitrogen concentration in the effluent could be constantly kept below 5 mg L(-1) N(tot), which provides the basis for being exempted from the wastewater discharge for nitrogen. As a result of all these measures, the rate of self-sufficiency by using biogas from the digester in combined heat and power units has been increased substantially from 60% before process modifications to 97%. With the upcoming optimisation measures, a further increase of self-sufficiency is expected to finally achieve energy neutrality based on yearly averages. The example of the WWTP Bochum-Ölbachtal clearly shows that it is worthwhile to consider further measures for improving the energy and treatment efficiency, although most of the machinery was not fully depreciated at the time of implementing the new concept. It is also evident that the regular energy monitoring is essential for identifying potential for improvement. In this context and based on Ruhrverband's experience, it is highly suggested that energy audits are performed regularly for larger WWTPs.

Experimental Study on Nitrogen Removal of Domestic Sewage Using Two-Stages Soil Aquifer Treatment System with Pre-Denitrification Process

For unstable high water ammonia shortcomings of soil infiltration system for rural sewage, one kind of new soil infiltration systems-two-stages soil aquifer treatment (SAT) system with pre-denitrification process was designed. The results show that when the hydraulic loading is within 0.06-0.08 m3/(m2·d), the effluent CODCr and NH4+-N concentration reach GB18918-2002 level A standard, the removal rate is 91%-94% for CODCr and 95%-98% for NH4+-N. The TN concentration is below 20 mg/L, meeting GB18918-2002 level B standard, and the removal rate is 71%-75%. The TN removal is dramatically affected by reflux ratio.

Full-scale in-line hydrolysis and simulation for potential energy and resource savings in activated sludge – a case study

The potential effects of altering primary settlers during biological in-line hydrolysis and converting a nitrifying activated sludge process into a partial pre-denitrification process for the purpose of resource conservation were evaluated. A full-scale primary sludge hydrolysis experiment was performed at a wastewater treatment plant and implemented in a dynamic modelling tool based on ASM2d. The full-scale hydrolysis experiment achieved a volatile fatty acid (VFA) production of 43 g CODHAc·m−3 with no release of ammonium. Additional nitrogen removal of 44 t N·a−1 was simulated, and the produced hydrolysate was able to replace 50% of the annual ethanol usage. Furthermore, 196 MWh of electricity per annum could be saved through the reduction of ethanol production and the optimization of the operation strategy of the activated sludge tank by operating a different number of anoxic zones.

Deterioration of denitrification by oxygen and cost evaluation of electron donor in an uncovered pre-denitrification process

Specific nitrate uptake rates (SNURs) under two test conditions were measured to evaluate effects of oxygen inhibition on denitrification. A test condition was that activated sludge was completely prevented from contacting of oxygen (SNURclosed), the other was that activated sludge was contacted to free air (SNURopen). Municipal wastewater and acetate were used as electron donors. SNURclosed was 2.42 mg NO3-N/g VSS-hr and SNURopen was 1.09 mg NO3-N/g VSS-hr when municipal wastewater was used as electron donor. Meanwhile, when acetate was used as electron donor, SNURclosed was 24.65 mg NO3-N/g VSS-hr and SNURopen was 18.00 mg NO3-N/g VSS-hr. The operating costs for electron donors were calculated based on the unit price of acetate to remove nitrate. When municipal wastewater was used as electron donor the ratio of costopen to costclosed was 0.45. Cost evaluation showed the adverse impacts on denitrification and explained why an anoxic reactor should be sequestered from oxygen.

Response of nitrifying bacterial communities to the increased thiocyanate concentration in pre-denitrification process

Changes in process performance and the nitrifying bacterial community associated with an increase of thiocyanate (SCN-) loading were investigated in a pre-denitrification process treating industrial wastewater. The increased SCN- loading led to the concentration of total nitrogen (TN) in the final effluent, but increasing the internal recycling ratio as an operation parameter from 2 to 5 resulted in a 21% increase in TN removal efficiency. In the aerobic reactor, we found that the Nitrosomonas europaea lineage was the predominant ammonia oxidizing bacteria (AOB) and the percentages of the AOB population within the total bacteria increased from about 4.0% to 17% with increased SCN- concentration. The increase of nitrite loading seemed to change the balance between Nitrospira and Nitrobacter, resulting in the high dominance of Nitrospira over Nitrobacter. Meanwhile, a Thiobacillus thioparus was suggested to be the main microorganism responsible for the SCN- biodegradation observed in the system.

Sudden failure of biological nitrogen and carbon removal in the full-scale pre-denitrification process treating cokes wastewater

A full-scale pre-denitrification process treating cokes wastewater containing toxic compounds such as phenols, cyanides and thiocyanate has shown good performance in carbon and nitrogen removal. However, field operators have been having trouble with its instability without being able to identify the causes. To clarify the main cause of these sudden failures of the process, comprehensive studies were conducted on the pre-denitrification process using a lab-scale reactor system with real cokes wastewater. First, the shock loading effects of three major pollutants were investigated individually. As the loading amount of phenol increased to 600 mg/L, more COD, TOC and phenol itself were flowed into the aerobic reactor, but phenol itself did not inhibit nitrification and denitrification, owing to the effect of dilution and its rapid biodegradation. Higher loading of ammonia or thiocyanate slightly enhanced the removal efficiency of organic matter, but caused the final discharge concentration of total nitrogen to be above its legal limit of 60 mg-N/L. Meanwhile, continuous inflow of abnormal wastewater collected during unstable operation of the full-scale pre-denitrification process, caused a sudden failure of nitrogen removal in the lab-scale process, like the removal pattern of the full-scale one. This was discovered to be due to the lack of inorganic carbon in the aerobic reactor where autotrophic nitrification occurs.

Effect of HRT on the biological pre-denitrification process for the simultaneous removal of toxic pollutants from cokes wastewater

A lab-scale serial anoxic–aerobic reactor for the pre-denitrification process was continuously operated to efficiently and economically treat actual cokes wastewater containing various pollutants, such as phenol, ammonia, thiocyanate and cyanide compounds. The biodegradation efficiencies of the pollutants were examined by changing hydraulic retention time (HRT) as a main operating variable. The long-term operation of the pre-denitrification process reactor showed that ∼100% phenol, ∼100% free cyanide, ∼100% SCN −, 97% ammonia, 85% COD, 84% TOC (total organic carbon) and 83% TN (total nitrogen) were removed at HRT above 11.9 h. Removal efficiency of total cyanides significantly decreased with a decrease in the HRT. Free cyanide and some of total cyanides were removed in anoxic reactor, whereas thiocyanate was removed in aerobic reactor. Phenol was completely removed under successive anoxic and aerobic conditions. Although actual cokes wastewater contained high concentrations of various toxic pollutants, the pre-denitrification process showed stable and successful performances in both nitrification and denitrification reactions.

Experimental study of nitrite accumulation in predenitrification biological nitrogen removal process

The effect of dissolved oxygen (DO) concentration on nitrite accumulation was investigated in a pilot-scale pre-denitrification process at room temperature for 100 days. In the first 10 days, due to the instability of the system, the DO concentration fluctuated between 1.0 and 2.0 mg/L. In the next 14 days, the DO concentration was kept at 0.5 mg/L and nitrite accumulation occurred, with the average nitrite accumulation rate at 91%. From the 25th day, the DO concentration was increased to 2.0 mg/L to destroy the nitrite accumulation, but nitrite accumulation rate was still as high as 90%. From the 38th day the nitrite accumulation rate decreased to 15%–30% linearly. From the 50th day, DO concentration was decreased to 0.5 mg/L to resume nitrite accumulation. Until the 83rd day the nitrite accumulation rate began to increase to 80%. Dissolved oxygen was the main cause of nitrite accumulation, taking into account other factors such as pH, free ammonia concentration, temperature, and sludge retention time. Because of the different affinity for oxygen between nitrite oxidizing bacteria and ammonia oxidizing bacteria when DO concentration was kept at 0.5 mg/L, nitrite accumulation occurred.

Chemical treatment for treating cyanides-containing effluent from biological cokes wastewater treatment process

A pre-denitrification process has been used to treat cokes wastewater containing toxic compounds such as phenols, cyanides and thiocyanate in Korea, and has showed very good removal efficiencies in carbon and nitrogen removals. However, a considerable amount of cyanides in the form of ferricyanide remained in the effluent of biological treatment process. Though ferrous iron is known to be more efficient in removing ferricyanide than ferric iron, ferric chloride solution has been used as a chemical precipitant due to its low cost for a long time. In this study, ferrous sulfate and ferric chloride solutions were used to remove cyanides remained in the effluent of the pre-denitrification process. The optimum dosage of each iron solution was evaluated in batch experiments with or without PAC solution. In addition, the amount of produced chemical sludge and the settling performance of it were also examined numerically. In conclusion, economic assessment indicated that ferrous iron is more economically profitable than ferric iron in spite of its high cost.

Inhibitory effects of toxic compounds on nitrification process for cokes wastewater treatment

Cokes wastewater is one of the most toxic industrial effluents since it contains high concentrations of toxic compounds such as phenols, cyanides and thiocyanate. Although activated sludge process has been adapted to treat this wastewater, nitrification process has been occasionally upset by serious inhibitory effects of toxic compounds. In this study, therefore, we examined inhibitory effects of ammonia, thiocyanate, free cyanide, ferric cyanide, phenol and p-cresol on nitrification in an activated sludge system, and then correlated their threshold concentrations with the full-scale pre-denitrification process for treating cokes wastewater. Ammonia below 350 mg/L did not cause substrate inhibition for nitrifying bacteria. Thiocyanate above 200 mg/L seemed to inhibit nitrification, but it was due to the increased loading of ammonia produced from its biodegradation. Free cyanide above 0.2 mg/L seriously inhibited nitrification, but ferric cyanide below 100 mg/L did not. Phenol and p-cresol significantly inhibited nitrification above 200 mg/L and 100 mg/L, respectively. Meantime, activated carbon was added to reduce inhibitory effects of phenol and free cyanide.

Bioaugmentation of cyanide-degrading microorganisms in a full-scale cokes wastewater treatment facility

To enhance biological removal efficiency of total cyanides, bioaugmentation was applied to a full-scale cokes wastewaters treatment process. After a laboratorial-scale cultivation (up to 1.2 m 3) of a cyanide-degrading yeast ( Cryptococcus humicolus) and unidentified cyanide-degrading microorganisms, the microbial consortium was inoculated into a fluidized-bed type process (1280 m 3), and then enriched for two months with a huge supply of glucose, KCN and other nutrients. Target wastewater was effluent of a biological pre-denitrification process for treating cokes wastewater, and contained about 14 mg/L of total cyanides in the form of ferric cyanide. This may be a first or rare report on the full-scale bioaugmentation of specialized-microorganisms. However, continuous operation of the full-scale cyanides-degrading bioprocess showed poor removal efficiency than expected owing to poor settling performance of microbial flocs, slow biodegradation rate of ferric cyanide and lack of organic carbon sources within the wastewater. Therefore, there is a need for further studies on how to solve these operating problems in full-scale bioaugmentation approach.

Denitrification potential enhancement by addition of external carbon sources in a pre-denitrification process

The aim of this study is to investigate the denitrification potential enhancement by addition of external carbon sources and to estimate the denitrification potential for the predenitrification system using nitrate utilization rate (NUR) batch tests. It is shown that the denitrification potential can be substantially increased with the addition of three external carbon sources, i.e. methanol, ethanol, and acetate, and the denitrification rates of ethanol, acetate, and methanol reached up to 9.6, 12, and 3.2 mgN/(g VSS·h), respectively, while that of starch wastewater was only 0.74 mgN/(g VSS·h). By comparison, ethanol was found to be the best external carbon source. NUR batch tests with starch wastewater and waste ethanol were carried out. The denitrification potential increased from 5.6 to 16.5 mg NO 3-N/L owing to waste ethanol addition. By means of NUR tests, the wastewater characteristics and kinetic parameters can be estimated, which are used to determine the denitrification potential of wastewater, to calculate the denitrification potential of the plant and to predict the nitrate effluent quality, as well as provide information for developing carbon dosage control strategy.

Comparison of nutrient removal efficiency between pre- and post-denitrification wastewater treatments

A shortage of organic substances (COD) may cause problems for biological nutrient removal, that is, lower influent COD concentration leads to lower nutrient removal rates. Biological phosphorus removal and denitrification are reactions in which COD is indispensable. As for biological simultaneous nitrogen and phosphorus removal systems, a competition problem of COD utilisation between polyphosphate accumulating organisms (PAOs) and non-polyphosphate-accumulating denitrifiers is not avoided. From the viewpoint of effective utilisation of limited influent COD, denitrifying phosphorus-removing organisms (DN-PAOs) can be effective. In this study, DN-PAOs activities in modified UCT (pre-denitrification process) and DEPHANOX (post-denitrification process) wastewater treatments were compared. In conclusion, the post-denitrification systems can use influent COD more effectively and have higher nutrient removal efficiencies than the conventional pre-denitrification systems.

Fuzzy control of nitrogen removal in predenitrification process using ORP

In order to meet increasingly stringent discharge standards, new applications and control strategies for the sustainable removal of nitrogen from wastewater have to be implemented. In the past years, numerous studies have been carried out dealing with the application of fuzzy logic to improve the control of the activated sludge process. In this paper, fuzzy control strategies of predenitrification systems are presented that could lead to better effluent quality and, in parallel, to a reduction of chemicals consumption. Extensive experimental investigations on lab scale plant studies have shown that there was excellent correlation between nitrate concentration and ORP value at the end of the anoxic zone. Results indicated that ORP could be used as an on-line fuzzy control parameter of nitrate recirculation and external carbon addition. The optimal value of ORP to control nitrate recirculation and external carbon addition was - 86 +/- 2 mV and - 90 +/- 2 mV, respectively. The results obtained with real wastewater also showed the good performance and stability of the fuzzy controllers independently from external disturbances. The integrated control structure of nitrate recirculation and external carbon addition in the predenitrification system is also presented.

New automatic control strategies for sludge recycling and wastage for the optimum operation of predenitrification processes

AbstractDynamic modelling and simulation is increasingly being employed as an aid in the design and operation of wastewater treatment plants (WWTPs). In this paper a simulation model is used to investigate the control of an activated sludge process. Two different control strategies to optimise nitrogen removal in a predenitrification process are presented, which are based on the control of the sludge blanket height (SBH) or sludge age (sludge retention time, SRT) and mixed liquor suspended solids (MLSS) in the reactor, by adjusting the sludge wastage and sludge recycle flow rates, respectively. It is shown how the recycle and wastage flow rate variables can be employed to continually maintain plant operation in the presence of disturbances. The performance of the control strategies has been assessed by the COST 624 benchmark plant and a pilot plant. The results have shown the robustness of the implementation and the efficiency of the proposed control strategies, which have been operated automatically in a safe, stable and optimum operating point, improving effluent quality and reducing energy costs. Consideration is also given as to how plant operation could be optimised in the long‐term in order to meet the yearly total nitrogen standard. Copyright © 2005 Society of Chemical Industry

A novel two-stage MBR denitrification process for the treatment of high strength pet food wastewater

A novel paradigm using pre-denitrification process is presented to optimize an existing system of two-stage MBRs treating high strength pet food wastewater. Successive reduction of organics in the 1st stage and almost complete nitrification in the 2nd stage generated effluent meeting stringent surface discharge criteria i.e. BOD 5, TSS and NH 4 +-N of <10 mg/L at an overall HRT of 6.3 days. Pre-anoxic zone was created by a submerged coil in the path of influent to the 1st stage. Final effluent and the 1st stage mixed liquor were recirculated to the coil. With prevailing high denitrification rates, more than 94% of the recirculated nitrates were denitrified in less than 15 min of effective anoxic residence time. At a recycle ratio of 3:1, total nitrogen was reduced by 84%, aeration energy by 25% and the external alkalinity requirement by 65%, enhancing economical viability of the system.

유기물부하가 낮은 하수의 전달탈질공법에 의한 탈질방안

This study was conducted to analyze the operating conditions of predenitrification process to improve the treatment efficiency in low organic loading sewage plant in use today, and to investigate the treatment efficiency of pilot plant added night soil as well as the nitrogen removal characteristics of pilot plant added carbon sources. In the operation under the condition of <TEX>$BOD_{5}$</TEX> sludge load 0.03-0.28kg <TEX>$BOD_{5}$</TEX>/kg VSS/d and oxic ammoniac nitrogen sludge load 0.02-0.24 <TEX>$kgNH_{4}^{+}$</TEX>-N/kg MLVSS/d, nitrification efficiency is higher than 95%. In order to achieve 70% nitrogen removal at the T-N sludge loading 0.06kg T-N/kg VSSㆍd and the SRT 6～11 days, optimum operating factors were revealed to <TEX>$CODc_{r}$</TEX>/T-N ratio 9, recycle ratio 2.6, and denitrification volume ratio 0.33. At this time, denitrification capacity was approximately 0.09 kg <TEX>$NO_{3}^{-}$</TEX>-N/kg <TEX>$CODc_{r}$</TEX>; specific nitrification rate was 3.4mg <TEX>$NH_{4}^{+}$</TEX>-N/g MLVSS/hr; and specific denitrification rate was 4.8mg <TEX>$NO_{3}^{-}$</TEX>-N/g MLVSS/hr.

Biological nitrification and denitrification of opto-electronic industrial wastewater

Development and application of biological nutrient removal processes accelerated significantly over the past decade due to more stringent nutrients (nitrogen and phosphorus) discharge limits being imposed on wastewater treatment plants. The opto-electronic industry has developed very fast over the past decade in the world. The wastewater often contains a significant quantity of organic nitrogen compounds and has a ratio of over 95% in organic nitrogen (Org-N) to total nitrogen (T-N). In this study, a 2-stage Anoxic/Aerobic pre-denitrification process was established and the efficiency of wastewater treatment was evaluated. Wastewater from an actual LCD-plant was obtained as the sample for looking into the feasibility of opto-electronic industrial wastewater treatment. Hydraulic retention time (HRT) and mixed liquor recycle rate (MLR) were controlled independently to distinguish between the effects of these two factors. Under suitable HRT and mixed liquor recycle ratio, effluents of NH4-N, NOx-N and COD can fall below 20 mg/l, 30 mg/l and 80 mg/l.

Biological nitrogen removal from landfill leachate: a pilot-scale study

A pilot-scale two-stage process including an anaerobic pretreatment (up-flow anaerobic sludge blanket, UASB) unit and a pre-denitrification activated sludge process was studied for treating leachates from a municipal landfill (waste aged 4 to 5 years, area 2.5 ha) and from a windrow composting area (1 ha). A single-stage pilot process was used for nitrification studies. The leachate characteristics were as follows: COD 740 to 2400 mg l–1 (chemical oxygen demand), BOD7 (biochemical oxygen demand) approximately 1000 mg l–1, Ntot 40 to 120 mg l–1 and the temperature was between 2 and 21°C. The results show that 45 to 50% COD removal could be achieved in the UASB reactor even at temperatures below 10°C. The activated sludge process was operated with a 1.1 to 2.2 d hydraulic retention time and at between 8 and 21°C. The removal efficiencies were 80 to 90% for COD, over 98% for BOD7, over 90% for NH4-N and over 70% for Ntot. The dependence of denitrification efficiency on leachate biodegradable COD fraction and COD to N ratio was evaluated. Overall treatment efficiencies were good and applicable for design purposes. The total costs were estimated for a full scale pre-denitrification process at FIM 10 to 20 m−3 and FIM 50 to 90 (kg N removed)−1 (3.5 to 7 D m m–3 and 17 to 30 D m [kg N removed]–1) for a landfill representing circumstances in Southern Finland with an annual average flow of 50 m3 d–1.