Anaerobic Retention Time Research Articles

Molecular evidence of internal carbon-driven partial denitrification in a mainstream pilot A-B system coupled with side-stream EBPR treating municipal wastewater

Achieving mainstream short-cut nitrogen removal via nitrite has become a carbon and energy efficient way, but still remains challenging for low-strength municipal wastewaters. This study integrated sidestream enhanced biological phosphorus removal system in a pilot-scale adsorption/bio-oxidation (A-B) process (named A-B-S2EBPR system) and nitrite accumulation was successfully achieved for treating the municipal wastewater. Nitrite could accumulate to 5.5 ± 0.3 mg N/L in the intermittently aerated tanks of B-stage with the nitrite accumulation ratio (NAR) of 79.1 ± 6.5 %. The final effluent concentration and removal efficiency of total inorganic nitrogen (TIN) were 4.6 ± 1.8 mg N/L and 84.9 ± 5.6 %, respectively. In-situ process performance of nitrogen conversions, routine batch nitrification/denitrification activity tests and functional gene abundance of nitrifiers collectively suggested that the nitrite accumulation was mainly caused by partial denitrification rather than out-selection of nitrite oxidizing bacteria (NOB). Moreover, the single-cell Raman spectroscopy analysis first demonstrated that there was a specific microbial population that could utilize polyhydroxyalkanoates (PHA) as the potential internal carbon source during the partial denitrification process. The integration of S2EBPR brings unique features to the conventional A-B process, such as extended anaerobic retention time, lower oxidation–reduction potential (ORP), much higher and complex volatile fatty acids (VFAs) etc., which can largely reshape the microbial communities. The dominant genera were Acinetobacter and Comamonadaceae, which accounted for (17.8 ± 15.5)% and (6.7 ± 3.4)%, respectively, while the relative abundance of conventional nitrifiers was less than 0.2%. This study provides insights into phylogenetic and phenotypic shifts of microbial communities when incorporating S2EBPR into the sustainable A-B process to achieve mainstream short-cut nitrogen removal.

Combined effects of volume ratio and nitrate recycling ratio on nutrient removal, sludge characteristic and microbial evolution for DPR optimization

The optimization of volume ratio (VAn/VA/VO) and nitrate recycling ratio (R) in a two-sludge denitrifying phosphorus removal (DPR) process of Anaerobic Anoxic Oxic-Moving Bed Biofilm Reactor (A2/O-MBBR) was investigated. The results showed that prolonged anaerobic retention time (HRTAn: 1.25→3.75 hr) exerted favorable effect on chemical oxygen demand (COD) removal (57.26%→73.54%), poly-β-hydroxyalkanoates (PHA) synthesis (105.70→138.12 mgCOD/L) and PO43− release (22.3→38.9 mg/L). However, anoxic retention time (HRTA) and R exhibited positive correlation with PHA utilization (43.87%-81.34%) and denitrifying phosphorus removal (DPR) potential (ΔNO3−/ΔPO43−: 0.57-1.34 mg/mg), leading to dramatical TN removal variations from 68.86% to 81.28%. Under the VAn/VA/VO ratio of 2:6:0, sludge loss deteriorated nutrient removals but the sludge bioactivity quickly recovered when the oxic zone was recovered. The sludge characteristic and microstructure gradually transformed under the dissolved oxygen (DO) control (1.0-1.5→1.5-2.0 mg/L), in terms of sludge volume index (SVI: 194→57 mL/gVSS), median-particle-size (D50: 99.6→300.5 μm), extracellular polymeric substances (EPS) (105.62→226.18 mg/g VSS) and proteins/polysaccharides (PN/PS) ratio (1.52→3.46). Fluorescence in situ hybridization (FISH) results showed that phosphorus accumulation organisms (PAOs) (mainly Cluster I of Accumulibacter, contribution ratio: 91.79%-94.10%) dominated the superior DPR performance, while glycogen accumulating organisms (GAOs) (mainly Competibacter, contribution ratio: 82.61%-86.89%) was responsible for deteriorative TN and PO43− removals. The optimal HRTA and R assembled around 5-6.5 hr and 300%-400% based on the PHA utilization and DRP performance, and the oxic zones also contributed to PO43− removal although it showed low dependence on DO concentration and oxic retention time (HRTO).

Assessment of sludge reduction and microbial dynamics in an OSA process with short anaerobic retention time

Abstract This study operated for 160 days and evaluated sludge minimization using an oxic-settling-anaerobic (OSA) system fed with real wastewater. At first, a pilot-scale conventional activated sludge (CAS) system was used to perform conventional wastewater treatment over a course of 60 days. Next, the CAS system was coupled with a sludge holding tank (SHT) in the sludge recirculation line to employ the OSA system, using a short 12h-hydraulic retention time (HRT) over a course of 100 days. The observed sludge yield ( Y obs ) dropped from 0.33 (CAS) to 0.19 (OSA) kg TSS kg−1 COD, ensuring a reduction of 43% of sludge in the OSA system. Better wastewater treatment performance was achieved using the OSA system (BOD5: 87%, TKN: 92%, NH 4 + -N: 94%) when compared with the CAS system (BOD5: 76%, TKN: 74%, NH 4 + -N: 78%), considering the organic matter and nitrogen removal rates. Moreover, the enrichment of anaerobic and hydrolyzing communities in the SHT (mainly Pedobacter and Acidovorax) enhanced the COD and nitrogen hydrolysis, which therefore improved the autotrophic metabolism in the aeration tank.

Side-stream enhanced biological phosphorus removal (S2EBPR) process improves system performance - A full-scale comparative study

To address the common challenges in enhanced biological phosphorus removal (EBPR) related to stability and unfavorable influent carbon to phosphorus ratio, a side-stream EBPR (S2EBPR) process that involves a side-stream anaerobic biological sludge hydrolysis and fermentation reactor was proposed as an emerging alternative. In this study, a full-scale pilot testing was performed with side-by-side operation of a conventional anaerobic-anoxic-aerobic (A2O) process versus a S2EBPR process. A comparison of the performance, activity and microbial community between the two configurations was performed. The results demonstrated that, with the same influent wastewater characteristics, S2EBPR configuration showed improved P removal performance and stability than the conventional A2O configuration, especially when the mixers in the side-stream anaerobic reactor were operated intermittently. Mass balance analysis illustrated that both denitrification and EBPR were enhanced in S2EBPR configuration, where return activated sludge was diverted into the anaerobic zone to promote fermentation and enrichment of polyphosphate accumulating organisms (PAOs), and the influent was bypassed to the anoxic zone for enhancing denitrification. A relatively higher PAO activity and total PAO abundance were observed in S2EBPR than in A2O configuration, accompanied by a higher degree of dependence on glycolysis pathway than tricarboxylic acid cycle. No significant difference in the relative abundances of putative PAOs, including Ca. Accumulibacter and Tetrasphaera, were observed between the two configurations. However, higher microbial community diversity indices were observed in S2EBPR configuration than in conventional one. In addition, consistently lower relative abundance of known glycogen accumulating organisms (GAOs) was observed in S2EBPR system. Extended anaerobic retention time and conditions that generate continuous and more complex volatile fatty acids in the side-stream anaerobic reactor of S2EBPR process likely give more competitive advantage for PAOs over GAOs. PAOs exhibited sustained EBPR activity and delayed decay under extended anaerobic condition, likely due to their versatile metabolic pathways depending on the availability and utilization of multiple intracellular polymers. This study provided new insights into the effects of implementing side-stream EBPR configuration on microbial populations, EBPR activity profiles and resulted system performance.

The Law of the Residual COD in Distilled Spirit Wastewater after Two Stages of Biochemical Treatment

Based on the tests of the high concentration organic wastewater from the sampling of Maotai flavor liquor enterprises in Chishui River Basin after two stages of biochemical treatment, this study found out the residue COD concentration range and investigated the rules of its formation. Under anaerobic conditions,the inert COD (SI) were measured.The experimental results showed that: the anaerobic retention time was 48 hours and the COD concentration of the effluent after aerobic biological treatment was about 300~350mg/L, the residual BOD5accounted for 1~2%, the difficult to degrade in raw water COD accounted for 28~33% and SMP accounted for 65~73%. In conclusion, the research findings can provide a scientific justification for the depth treatment process of liquor wastewater to reach the standard.

Biological phosphorus removal from dairy wastewater by alternating anaerobic and aerobic conditions

In this study, the possibility of applying the enhanced biological phosphorus removal (EBPR) process for Algiers dairy wastewater which can have phosphorus contents up to 130 mg/L was examined. EBPR is conventionally performed by an anaerobic-aerobic process. The objectives of this work were to determine an optimal hydraulic retention time (HRT) in aerobic conditions and to study the effect of short chain fatty acids (SCFA) (acetic and propionic acids) addition on the phosphorus release in anaerobic conditions. The tests were performed in a batch reactor operating with an aerobic/anaerobic sequence of phases. Batch tests have been carried out at 3 HRTs in aerobic conditions (1, 2 and 3 h) while the anaerobic retention time was fixed at 4 h, to examine the effect of stress related to changes of aerobic HRT. Main results show that the most favorable aerobic retention time was found to be 2 h. The amount of P released in anaerobic phase increases from 2.25 to 2.48 mgP/gVSS with increasing aerobic HRT from 1 to 2 h and decreases to 1.28 mgP/g VSS for a time of 3 h using acetic acid. Similarly, this amount increases from 1.62 to 4.38 mgP/gVSS for 1 to 2 h and decreases to 1.41 mgP/gVSS for a time of 3 h using propionic acid. The initial release rate was directly proportional to the amount of added substrate. Propionate may be a more effective carbon source for biological phosphorus removal than acetate. Based on the results presented herein, we can confirm the possibility of phosphorus removal from dairy wastewater in the aerobic-anaerobic biological process. Keywords: Dairy wastewater, biological phosphorus removal, aerobic, anaerobic, release, acetic acid, propionic acid

Effect of basic operating parameters on biological phosphorus removal in a continuous-flow anaerobic–anoxic activated sludge system

A continuous-flow anaerobic-anoxic (A2) activated sludge system was operated for efficient enhanced biological phosphorus removal (EBPR). Because of the system configuration with no aeration zones, phosphorus (P) uptake takes place solely under anoxic conditions with simultaneous denitrification. Basic operating conditions, namely biomass concentration, influent carbon to phosphorus ratio and anaerobic retention time were chosen as variables in order to assess their impact on the system performance. The experimental results indicated that maintenance of biomass concentration above 2,500mg MLVSS/L resulted in the complete phosphate removal from the influent (i.e. 15mg PO(4) (3-)-P/L) for a mean hydraulic residence time (HRT) of 15h. Additionally, by increasing the influent COD/P ratio from 10 to 20g/g, the system P removal efficiency was improved although the experimental results indicated a possible enhancement of the competition between phosphorus accumulating organisms (PAOs) and other microbial populations without phosphorus uptake ability. Moreover, because of the use of acetate (i.e. easily biodegradable substrate) as the sole carbon source in the system feed, application of anaerobic retention times greater than 2h resulted in no significant release of additional P in the anaerobic zone and no further amelioration of the system P removal efficiency. The application of anoxic P removal resulted in more than 50% reduction of the organic carbon necessitated for nitrogen and phosphorus removal when compared to a conventional EBPR system incorporating aerobic phosphorus removal.

Compartmentalizing anaerobic zone improves sludge settling property of enhanced biological phosphorus removal system under low sludge retention time

The study investigated the effect of compartmentalization of anaerobic zone on sludge settling property in an enhanced biological phosphorus removal (EBPR) system. Continuous‐flow pilot‐plant experiments with synthetic influent were performed at varied anaerobic‐oxic volume ratios (Van : Vo , 1 : 9 and 2 : 8), involving 5, 10 and 15 days of sludge retention times (SRTs). Experimental results indicated that, regardless of Van : Vo and SRT, satisfactory removal of COD and phosphate were achieved during all test runs. However, extremely high sludge volume index (SVI) values were observed when there were 5 days of SRT. Further experiments with compartmentalization of anaerobic zone were therefore conducted to confirm the necessity of incorporating the kinetic selection mechanism of bulking control into an EBPR process operated at a low SRT. These results revealed that the two factors, Van : Vo and anaerobic compartmentalization, had interaction with respect to sludge settling property in terms of SVI. When anaerobic retention time increased from 0.72hr to 1.36hr, compartmentalizing anaerobic zone significantly decreased the SVI value because of generating high COD concentration gradient and phosphate release in the anaerobic zone. This result implies that supplementing kinetic selection pressure of bulking control exhibits a critical influence on sludge settling when the EBPR process operates at a lower SRT.

Short-Term Effects of Wastewater Biodegradability on Biological Phosphorus Removal

A steady-state laboratory-scale sequencing batch reactor process for biological phosphorus removal (BPR) was developed, and the influence of wastewater biodegradability on BPR was studied in batch tests. Biodegradability was expressed in this work as the readily biodegradable fraction of wastewater COD (rbCOD) present in the mixed liquor after the anaerobic stage of the anaerobic/oxic cycle of the BPR process. The rbCOD fraction was changed by varying the organic composition of synthetic wastewater (different carbohydrates were used: saccharose, cellobiose, starch, and cellulose) or varying the anaerobic retention time (1.25, 4, 9, and 24 h) when only one kind of low biodegradable synthetic wastewater (starch composed) was used. A clear positive trend was observed between rbCOD and anaerobic P release, but such a clear relationship was not observed with BOD\d5 measurements. Soluble carbohydrates allowed a BPR mechanism, but particulate carbohydrates seemed to cause nonbiological P removal. An increase in anaerobic retention time improved rbCOD concentrations up to 50%, approximately, and P removal, but excessive retention times, >9 h, should be used to reach good BPR results.

Respirometric determination of the readily biodegradable cod produced in the anaerobic stage of a biological phosphorus removal process

A respirometric method for determination of the readily biodegradable chemical oxygen demand (COD) concentration in a wastewater was developed, and used to study the effect of the anaerobic stage of a biological phosphorus (P) removal process. The sludge used was obtained from a laboratory scale activated sludge A/O (anaerobic/aerobic) process for biological P removal, fed with synthetic wastewater composed by glucose and peptone as carbon sources. Respirometric tests were made in an electrolytic respirometer. Wastewater short‐time biochemical oxygen demands were measured after anaerobic reactions using the sludge from the A/O process, and the readily biodegradable COD concentrations were then calculated. Several tests were made using different kinds of synthetic wastewater in order to know the accuracy of the experimental technique. These wastewaters were prepared using a constant COD level but different organic composition in order to change their biodegradability. The method was used to study the effect of the anaerobic period duration on the readily biodegradable COD concentration produced from a starch containing wastewater. A higher anaerobic retention time caused an increase in the readily biodegradable COD concentration. This determination can be useful to know the effectiveness of the anaerobic period and thus, the viability of the Biological Phosphorus Removal (BPR) process with slowly biodegradable wastewater.

Aerobic and anaerobic digestion of processed municipal solid waste

Effective disposal of municipal solid wastes (MSW) through biological processes must focus on the maximal reduction in bulk. Since cellulose is a major component in processed municipal solid waste, biological treatment processes must result in near complete degradation of the polymer. Specific cellulose degradation rates were determined for aerobic and anaerobic digestion systems fed a high cellulose, processed MSW feed, while maintaining optimum nutrient levels, mesophilic temperature (37°C), and a pH of 7.0–7.4. Reducing the retention time resulted in reduced cellulose conversion for both aerobic and anaerobic systems with similar trends. Anaerobic retention times of less than 12 d resulted in unstable digestion, with increased volatile fatty acid pools and decreases in pH. Reducing aerobic retention times below 8 d dramatically reduced cellulose conversion without unstable digestion conditions or volatile fatty acid accumulation.

Phosphate Removal at Pinetown's Umhlatuzana Plant: Acinetobacter and Ferrous Ions in Anaerobic Zones

The principal results of many years of study of the removal of nutrients from municipal wastewater by a full-scale mainstream activated sludge process (capable of 95% removal of C, N and P without chemical dosing) indicate that anaerobic zones have two important functions, the second of which has not been reported previously in wastewater research publications:(1) Anaerobic zones provide ideal conditions for uptake of organic carbon and release of phosphate by Acinetobacter which accounted for about 50% of the heterotrophic organisms in the plant as determined by the API system.(2) Anaerobic zones also provide ideal conditions for reduction of influent ferric iron to ferrous iron and subsequent precipitation and adsorption of mixed metallic phosphates; such as a colloidal modification of ferrous calcium phosphate (1;0,4:1 molar) which forms in the pH range 6 to 7,5. It was found that anaerobic retention times of short duration were sufficient to stimulate subsequent uptake of soluble phosphate by Acinetobacter in aerobic zones. However, much longer anaerobic retention times were necessary to lower the redox pote ntial sufficiently to reduce bound iron in the ferric state to ferrous ions which were identified in significant quantities in the sludge by using cold PCA fractionation procedures in conjunction with bathophenanthroline. The partial transfer in anaerobic zones of phosphate, from intracellular polyphosphate in vivo to extracellularly adsorbed metallic phosphates in vitro, was found to be essential for truly enhanced removal of phosphate by this mainstream process which always removed more than 95% of the nitrogen from the municipal wastewater. A further finding of significance was the limited phosphate storage capacity of Acinetobacter. Metabolic phosphate, excess biological uptake and extracellular precipitation/ adsorption each accounted for about one third of the total phosphate removed. About two thirds of the total influent nitrogen was removed by “simultaneous” nitrification and denitrification and most of the remainder was removed as organic nitrogen in the surplus sludge. The continuous recycling of activated sludge containing extracellularly adsorbed colloidal phosphate precipitates eliminates the need for nucleation and induction and provides additional surface area with active sites for adsorption of excess phosphate. This could explain the observation that phosphate removal efficiencies improved when solids retention times and sludge concentrations were increased. The plant and its performance have been described in detail in previous papers by the same Authors:1980 “The ability of the extended aeration activated sludge process to remove phosphorus consistently to less than 0,1 mg P/l in a simple surface-aerated rectangular reactor.” IWPC (South African Branch) Conference, Pretoria (2-6 June).1981 “Application of numerical models to design and operation of municipal wastewater treatment works incorporating multi-mini-step activated sludge reactors for enhanced P-removal.” Unpublished.1982 “Full scale phosphate removal experiences in the Umhlatuzana Works at different sludge ages.” IAWPR Post Conference Seminar on phosphate removal in biological treatment processes, Pretoria 5-6 ApriI 1982. Wat.Sci.Tech.Vol. 15, Cape Town, pp 261-281.1983 “Further developments in the understanding of phosphate removal at Umhlatuzana.” IWPC Conference, East London. Operating at sludge temperatures in the range 17°C to 26°C for many years without chemical dosing, the plant exhibited, from time to time, the ability to remove consistently for many consecutive months 95% of the carbon, phosphate, and nitrogen from unsettled municipal wastewater (in a soft water area), with a TKN/COD ratio of 0,07; a P/COD ratio of 0,014; and a low alkalinity/COD ratio of 0,25 (as CaCO3/COD).

The Influence of Extended Anaerobic Retention Time on the Performance of Phoredox Nutrient Removal Plants

Laboratory-scale studies on the performance of three-stage Phoredox nutrient removal systems under nominal anaerobic retention times varying from 6 to 24 h are described. Particular attention is given to the effect of these extended periods of anaerobiosis on biological phosphate release and uptake, nitrification and denitrification. Increasing the retention time in the abovementioned range had no deleterious effect on nitrification, COD removal, sludge settleability and denitrification but resulted in significantly improved phosphate removals. Up to 15 mg/ℓ phosphate (as P) could be removed from settled wastewater at nominal anaerobic retention times of 12 h or more.

The Role of Micro-Organisms Other than Acinetobacter in Biological Phosphate Removal in Activated Sludge Processes

The composition of the microflora in the anaerobic, anoxic and aerated stages was determined by means of the Analytical Profile Index (API) system for three biological phosphate removal plants (a pilot plant (capacity 100 m3/d) and two laboratory scale units (capacity 24 ℓ/d)). The plants monitored differed with respect to anaerobic retention time and configuration, but all three were removing phosphate to concentrations less than 0.2 mg/ℓ as P during the experimental period. In contrast to findings by other investigators, bacteria of the genus Acinetobacter were present in minor proportions. Organisms of the genera Aeromonas and Pseudomonas constituted more than 50 % of the total aerobic microbial population. Depending on the length of the anaerobic retention time, 40 to 50 % of the bacteria were identified as Gram-positive. These findings suggest that Acinetobacter species may not be the only bacterium to remove phosphate from an activated sludge system, but that other groups of bacteria also do so under suitable environmental conditions.