Removal of refractory compounds from stabilized landfill leachate using an integrated H2O2 oxidation and granular activated carbon (GAC) adsorption treatment

This study investigated the treatment performances of H₂O₂ oxidation alone and its combination with granular activated carbon (GAC) adsorption for raw leachate from the NENT landfill (Hong Kong) with a very low biodegradability ratio (BOD₅/COD) of 0.08. The COD removal of refractory compounds (as indicated by COD values) by the integrated H₂O₂ and GAC treatment was evaluated, optimized and compared to that by H₂O₂ treatment alone with respect to dose, contact time, pH, and biodegradability ratio. At an initial COD concentration of 8000 mg/L and NH₃-N of 2595 mg/L, the integrated treatment has substantially achieved a higher removal (COD: 82%; NH₃-N: 59%) than the H₂O₂ oxidation alone (COD: 33%; NH₃-N: 4.9%) and GAC adsorption alone (COD: 58%) at optimized experimental conditions (p ≤ 0.05; t-test). The addition of an Fe(II) dose at 1.8 g/L further improved the removal of refractory compounds by the integrated treatment from 82% to 89%. Although the integrated H₂O₂ oxidation and GAC adsorption could treat leachate of varying strengths, treated effluents were unable to meet the local COD limit of less than 200 mg/L and the NH₃-N of lower than 5 mg/L. However, the integrated treatment significantly improved the biodegradability ratio of the treated leachate by 350% from 0.08 to 0.36, enabling the application of subsequent biological treatments for complementing the degradation of target compounds in the leachate prior to their discharge.     

Biological nitrogen removal with a real-time control strategy using moving slope changes of pH(mV)- and ORP-time profiles

A new real-time control strategy using moving slope changes of oxidation-reduction potential (ORP)- and pH(mV)-time profiles was designed. Its effectiveness was evaluated by operating a farm-scale sequencing batch reactor (SBR) process using the strategy. The working volume of the SBR was 18 m³, and the volumetric loading rate of influent was 1 m³ cycle⁻¹. The SBR process comprised six phases: feeding → anoxic → anaerobic → aerobic → settle → discharge. The anoxic and aerobic phases were controlled by the developed real-time control strategy. The nitrogen break point (NBP) in the pH(mV)-time profile and the nitrate knee point (NKP) in the ORP-time profile were designated as real-time control points for the aerobic and anoxic phases, respectively. Through successful real-time control, the duration of the aerobic and anoxic phases could be optimized and this resulted in very high N removal and a flexible hydraulic retention time. Despite the large variation in the loading rate (0.5-1.8 kg NH₄-N m⁻³ cycle⁻¹) due to influent strength fluctuation, complete removal of NH₄-N (100%) was always achieved. The removal efficiencies of soluble nitrogen (NH₄-N plus NO_x-N), soluble total organic carbon, and soluble chemical oxygen demand were 98%, 90%, and 82%, respectively. Monitoring the ORP and pH(mV) revealed that pH(mV) is a more reliable control parameter than ORP for the real-time control of the oxic phase. In some cases, a false NBP momentarily appeared in the ORP-time profile but was not observed in the pH(mV)-time profile. In contrast, ORP was more the reliable control parameter for NKP detection in the anoxic phase, since the appearance of NKP in the pH(mV)-time profile was sometimes vague.     

Interaction between temperature and ammonia in mesophilic digesters for animal waste treatment

Four anaerobic sequencing batch reactors (ASBRs) were operated during a period of 988 days to evaluate the effect of temperature, ammonia, and their interconnectivity on the methane yield of anaerobic processes for animal waste treatment. During period 1 (day 0-378), the methane yield was 0.31 L CH₄/g volatile solids (VS) for all digesters (with no statistical differences among them) at a temperature and total ammonium-N levels of 25 °C and ∼1200 mg NH₄⁺-N/L, respectively. During period 2 (day 379-745), the methane yield at 25 °C decreased by 45% when total ammonium-N and ammonia-N were increased in two of the four ASBRs to levels >4000 mg NH₄⁺-N/L and >80 mg NH₃-N/L, respectively. During period 3 (day 746-988), this relative inhibition was reduced from 45% to 13% compared to the low-ammonia control reactors when the operating temperature was increased from 25 °C to 35 °C (while the free ammonia levels increased from ∼100 to ∼250 mg NH₃-N/L). The 10 °C increase in temperature doubled the rate constant for methanogenesis, which overwhelmed the elevated toxicity effects caused by the increasing concentration of free ammonia. Thus, the farmer/operator may alleviate ammonia toxicity by increasing the operating temperature within the mesophilic range. We extrapolated our data to correlate temperature, ammonia, and methane yield and to hypothesize that the difference between high- and low-ammonia reactors is negligible at the optimum mesophilic temperature of 38 °C.     

Treating landfill leachate using passive aeration trickling filters; effects of leachate characteristics and temperature on rates and process dynamics

Biological ammoniacal-nitrogen (NH₄⁺-N) and organic carbon (TOC) treatment was investigated in replicated mesoscale attached microbial film trickling filters, treating strong and weak strength landfill leachates in batch mode at temperatures of 3, 10, 15 and 30 °C. Comparing leachates, rates of NH₄⁺-N reduction (0.126-0.159 g m^− 2 d^− 1) were predominantly unaffected by leachate characteristics; there were significant differences in TOC rates (0.072-0.194 g m^− 2 d^− 1) but no trend relating to leachate strength. Rates of total oxidised nitrogen (TON) accumulation (0.012-0.144 g m^− 2 d^− 1) were slower for strong leachates. Comparing temperatures, treatment rates varied between 0.029-0.319 g NH₄⁺-N m^− 2 d^− 1 and 0.033-0.251 g C m^− 2 d^− 1 generally increasing with rising temperatures; rates at 3 °C were 9 and 13% of those at 30 °C for NH₄⁺-N and TOC respectively. For the weak leachates (NH₄⁺-N < 140 mg l^− 1) complete oxidation of NH₄⁺-N was achieved. For the strong leachates (NH₄⁺-N 883-1150 mg l^− 1) a biphasic treatment response resulted in NH₄⁺-N removal efficiencies of between 68 and 88% and for one leachate no direct transformation of NH₄⁺-N to TON in bulk leachate. The temporal decoupling of NH₄⁺-N oxidation and TON accumulation in this leachate could not be fully explained by denitrification, volatilisation or anammox, suggesting temporary storage of N within the treatment system. This study demonstrates that passive aeration trickling filters can treat well-buffered high NH₄⁺-N strength landfill leachates under a range of temperatures and that leachate strength has no effect on initial NH₄⁺-N treatment rates. Whether this approach is a practicable option depends on a range of site specific factors.     

Biochemical oxygen demand and nutrient processing in a novel multi-stage raw municipal wastewater and acid mine drainage passive co-treatment system

A laboratory-scale, four-stage continuous flow reactor system was constructed to test the viability of high-strength acid mine drainage (AMD) and municipal wastewater (MWW) passive co-treatment. The synthetic AMD had pH 2.60 and 1860 mg/L acidity as CaCO₃ equivalent with 46, 0.25, 2, 290, 55, 1.2 and 390 mg/L of Al, As, Cd, Fe, Mn, Pb and Zn, respectively. The AMD was introduced to the system at a 1:2 ratio with raw MWW from the City of Norman, Oklahoma USA containing 265 ± 94 mg/L BOD₅, 11.5 ± 5.3 mg/L PO₄⁻³, and 20.8 ± 1.8 mg/L NH₄⁺-N. During the 135 d experiment, PO₄⁻³ and NH₄⁺-N were decreased to <0.75 and 7.4 ± 1.8 mg/L, respectively. BOD₅ was generally decreased to below detection limits. Nitrification increased NO₃⁻ to 4.9 ± 3.5 mg/L NO₃⁻-N, however relatively little denitrification occurred. Results suggest that the nitrogen processing community may require an extended period to mature and reach full efficiency. Overall, results indicate that passive AMD and MWW co-treatment is a viable ecological engineering approach for the developed and developing world that can be optimized and applied to improve water quality with minimal use of fossil fuels and refined materials.     

Evaluating the influence of process parameters on soluble microbial products formation using response surface methodology coupled with grey relational analysis

Soluble microbial products (SMPs) present a major part of residual chemical oxygen demand (COD) in the effluents from biological wastewater treatment systems, and the SMP formation is greatly influenced by a variety of process parameters. In this study, response surface methodology (RSM) coupled with grey relational analysis (GRA) method was used to evaluate the effects of substrate concentration, temperature, NH₄⁺-N concentration and aeration rate on the SMP production in batch activated sludge reactors. Carbohydrates were found to be the major component of SMP, and the influential priorities of these factors were: temperature > substrate concentration > aeration rate > NH₄⁺-N concentration. On the basis of the RSM results, the interactive effects of these factors on the SMP formation were evaluated, and the optimal operating conditions for a minimum SMP production in such a batch activated sludge system also were identified. These results provide useful information about how to control the SMP formation of activated sludge and ensure the bioreactor high-quality effluent.     

Limited filamentous bulking in order to enhance integrated nutrient removal and effluent quality

Limited filamentous bulking has been proposed as a means to enhance floc size and make conditions more favorable for simultaneous nitrification/Denitrification (SND). Moreover a slightly heightened SVI is supposed to increase the removal of small particulates in the clarifier. Integrated nitrogen, phosphorus and COD removal performance under limited filamentous bulking was investigated using a bench-scale plug-flow enhanced biological phosphorus removal (EBPR) reactor fed with raw domestic wastewater. Limited filamentous bulking in this study was mainly induced by low DO levels, while other influencing factors associated with filamentous bulking (F/M, nutrients, and wastewater characteristics) were not selective for filamentous bacteria. The optimum scenario for integrated nitrogen, phosphorus and COD removal was achieved under limited filamentous bulking with an SVI level of 170-200 (associated with a DO of 1.0-1.5 mg/L). The removal efficiencies of COD, TP and NH₄⁺-N were 90%, 97% and 92%, respectively. Under these conditions, the solid-liquid separation was practically not affected and sludge loss was never observed. A well-clarified effluent with marginal suspended solids was obtained. The results of this study indicated the feasibility of limited filamentous bulking under low DO as a stimulation of simultaneous nitrification/denitrification for enhancing nutrient removal and effluent quality in an EBPR process.     

Influence of sand layer depth and percolate impounding regime on nitrogen transformation in vertical-flow constructed wetlands treating faecal sludge

Four laboratory-scale units of vertical-flow constructed wetlands (VFCW) were fed once a week with faecal sludge (FS) at a constant solids loading rate (SLR) of 250 kg TS/(m².year) (equivalent to 260-300 g N/(m².week)) for a period of 12 weeks to study: i) the nitrification and denitrification potential of the sand layer of VFCWs and ii) the effect of percolate impounding regime (permanent or batch-impounding) on nitrogen transformation. The TN content of raw FS was characterised by 65% org-N, 34% NH₄-N and 1% NO_x-N. After FS application and a six-day impounding period, 8-13% TN were recovered in the percolate exhibiting the following composition: 70-80% NH₄-N, 25-30% org-N and <1% NO_x-N. A large fraction of the influent organic N (55%) was filtered in the bed and 24-29% of initial NH₄-N were lost due to nitrification and volatilisation. In permanent impounding systems, 8-11% TN were recovered in the percolate versus 13% in batch-operated beds. N loss was increased with sand layer depth (20-40 cm) under permanent impounding regimes.     

Simultaneous removal of carbon and nutrients from an industrial estate wastewater in a single up-flow aerobic/anoxic sludge bed (UAASB) bioreactor

Simultaneous removal of carbon and nutrients (CNP) in a single bioreactor is highly significant for energy consumption and control of reactor volume. Basically, nutrients removal is dependant to the ratio of biochemical oxygen demand to chemical oxygen demand (BOD₅/COD). Thus, in this study the treatment of an industrial estate wastewater with low BOD₅/COD ratio in an up-flow aerobic/anoxic sludge bed (UAASB) bioreactor, with an intermittent regime in aeration and discharge, was investigated. Hydraulic retention time (HRT) of 12-36 h and aeration time of 40-60 min/h were selected as the operating variables to analyze, optimize and model the process. In order to analyze the process, 13 dependent parameters as the process responses were studied. From the results, it was found, increasing HRT decreases the CNP removal efficiencies. However, by increasing the BOD₅ fraction of the feed, the total COD (TCOD), slowly biodegradable COD (sbCOD), readily biodegradable COD (rbCOD), total nitrogen (TN), and total phosphorus (TP) removal efficiencies were remarkably increased. Population of heterotrophic, nitrifying and denitrifying bacteria showed good agreement with the results obtained for TCOD and TN removal. The optimum conditions were determined as 12-15 h and 40-60 min/h for HRT and aeration time respectively.     

Optimization of a full‐scale site to achieve total nitrogen removal through implementation of a denitrification‐submerged anoxic filter

A full‐scale wastewater treatment plant with a 5500 population equivalent was retrofitted with a pre‐denitrification‐submerged anoxic filter (SAnoF) in order to achieve new total nitrogen (TN) consent of 35 mg/L. A 36 m³ SAnoF was installed downstream the primary settling tanks. The optimal operation of the anoxic SAnoF was investigated by varying the recirculation ratio, the carbon‐to‐nitrate ratio, and the hydraulic retention time. After stable operation was achieved, nitrate was removed by 80% at a loading of 0.5 kg NO₃/m³ day and a retention time of 60 min. The SAnoF presented a number of advantages, including the use of internal carbon for denitrification, decrease of carbon load to the trickling filter by 30%, and production of alkalinity required for nitrification in the trickling filter (11 mg CaCO₃/mg NH₄ removed). Overall, the SAnoF was satisfactory and the effluent TN concentration reached 20–25 mg TN/L.