Low Mixed Liquor Suspended Solids Research Articles

Insights into the adsorption of perfluoroalkyl substances (PFASs) on excess sludge from an on-site leachate treatment plant

The disposal and reduction of excess sludge is one of the most challenging aspects of wastewater treatment. Recycling excess sludge as functional materials is an innovative approach that has garnered broad attention, as it can address environmental concerns and reduce the consumption of valuable materials. However, this approach is still challenging due to the low mixed liquor suspended solids in sewage treatment plants and the high synthesis costs involved. In this study, we proposed the application of excess sludge from an on-site leachate treatment plant for the efficient removal of perfluoroalkyl substances (PFASs) from wastewater. A simple thermal treatment was conducted and the obtained sludge-based adsorbents enable effective PFASs removal. Through the use of low field nuclear magnetic resonance and structural characterizations, results demonstrated that denitrifying sludge had higher hydrophobicity and specific surface area (30.85 m2/g) than nitrifying sludge, resulting in enhanced PFASs adsorption capacity and initial adsorption rates. Kinetic and isotherm simulations indicated that the adsorption was efficient and controlled by surface and intraparticle diffusion. Characterization and regulation results showed that the adsorption was dominated by hydrophobic and electrostatic interactions. These findings suggest a promising recycling approach of excess sludge while elucidating the dominant mechanisms involved.

One-stage partial nitrification/anammox system with short sludge retention time and low mixed liquor suspended solids long-term control: Establishment, performance and sludge characteristics

Anammox is a green, economical and efficient nitrogen removal process. Most successful anammox studies are based on biofilm- or granule-based systems, but pure floc sludge partial nitrification (PN) and anammox (A) systems that are not inoculated with anaerobic ammonia oxidizing bacteria (AnAOB) are rarely reported. If the anammox process occurs in floc-based systems, the large specific surface areas provide more efficient nitrogen removal, and are much more economical in terms of construction and investment. This study investigated the establishment, performance and sludge characteristics of a one-stage PN/A system with pure floc sludge and exhibited a short sludge retention time (SRT) and low mixed liquor suspended solids (SS) content. The experiment was run for approximately 1260 days and divided into five stages based on the SRTs and influent ammonia concentrations treating synthetic wastewater with no organic matter. The results showed that the AnAOB were successfully cultivated and enriched with ordinary nitrification and denitrification sludge, which formed a pure floc-based anammox system with a short SRT (at least 14 days) and a low SS control. The maximum nitrogen removal efficiency and sludge removal loading rate reached 87.1 % and 3.16 kg N/(kg VSS·d) with ammonia loading rates = 0.55 and 0.56 kg-N/(m3·d), dissolved oxygen = 0.2 and 0 mg/L, temperature = 30 and 28 °C, mixed liquor volatile suspended solid (VSS) = 800 and 130 mg/L, free ammonia (FA)/VSS = 3.5 and 47.5 mg NH3-N/g VSS and SRT = 30 and 15 days, respectively. Moreover, the FA/VSS ratio was used to determine the operating performance of the PN/A system, and the thresholds for inhibiting nitrite-oxidizing bacteria and ammonia-oxidizing bacteria, including AnAOB, were 0.5–50 and above 50 mg NH3-N/g VSS, respectively. The floc-based one-stage PN/A systems proposed in this study provide reductions in the volumes, and floor areas for the reactor tanks, and in the cost of the carrier.

Impact of different inoculum sources on the performance of membrane bioreactors for municipal wastewater treatment: Dynamic membrane versus ultrafiltration membrane

In this study, impact of different inoculum on the performance of membrane bioreactors (MBRs) was investigated for municipal wastewater treatment. Waste sludges from a conventional activated sludge (CAS) system and a high-rate activated sludge (HRAS) system were used as inoculums at Stage-1 and Stage-2, respectively. A commercial ultrafiltration (UF) membrane was tested in parallel with a low-cost polyester hollow fiber support material (dynamic membrane, DM). UF and DM membranes were operated for 67 days at a flux of 8 L/m2·h at each stage. High chemical oxygen demand (COD) and soluble COD (sCOD) removal efficiencies (>86% and >74%, respectively), low mixed liquor suspended solids (MLSS) concentration (<10 mg/L), and low turbidity values were achieved in permeates at each stage. Extracellular polymeric substances (EPS) content was higher at Stage-1, which caused an increase in sludge resistance to filtration (SRF) and sludge volume index (SVI). Based on morphological analysis, compact dynamic cake layer was formed on the support material at Stage-1, while porous dynamic layer was formed at Stage-2. Thus, DM was operated at lower TMP at Stage-2. Based on the results obtained from the study, sludge from HRAS system showed better performance as inoculum compared to the sludge from CAS system for DM applications.

Limitations imposed by conventional fine bubble diffusers on the design of a high-loaded membrane bioreactor (HL-MBR).

The operation of membrane bioreactors (MBRs) at higher than usual mixed liquor suspended solids (MLSS) concentrations may enhance the loading rate treatment capacity while minimizing even further the system's footprint. This requires operating the MBR at the highest possible MLSS concentration and biomass activity (e.g., at high loading rates and low solid retention times (SRTs)). Both a negative effect of the MLSS concentrations and a positive effect of the SRT on the oxygen transfer have been reported when using conventional fine bubble diffusers. However, most of the evaluations have been carried out either at extremely high SRTs or at low MLSS concentrations eventually underestimating the effects of the MLSS concentration on the oxygen transfer. This research evaluated the current limitations imposed by fine bubble diffusers in the context of the high-loaded MBR (HL-MBR) (i.e., high MLSS and short SRT-the latter emulated by concentrating municipal sludge from a wastewater treatment plant (WWTP) operated at a short SRT of approximately 5days). The high MLSS concentrations and the short SRT of the original municipal sludge induced a large fraction of mixed liquor volatile suspended solids (MLVSS) in the sludge, promoting a large amount of sludge flocs that eventually accumulated on the surface of the bubbles and reduced the free water content of the suspension. Moreover, the short SRTs at which the original municipal sludge was obtained eventually appear to have promoted the accumulation of surfactants in the sludge mixture. This combination exhibited a detrimental effect on the oxygen transfer. Fine bubble diffusers limit the maximum MLSS concentration for a HL-MBR at 30gL-1; beyond that point is either not technically or not economically feasible to operate; an optimum MLSS concentration of 20gL-1 is suggested to maximize the treatment capacity while minimizing the system's footprint.

Evaluating the treatment performance of a full scale Activated Sludge Plant in Islamabad, Pakistan

In this study, the performance of wastewater treatment plant located at sector I-9 Islamabad, Pakistan, was evaluated. This full scale domestic wastewater treatment plant is based on conventional activated sludge process. The parameters which were monitored regularly included total suspended solids (TSS), mixed liquor suspended solids (MLSS), mixed liquor volatile suspended solids (MLVSS), biological oxygen demand (BOD), and chemical oxygen demand (COD). It was found that the biological degradation efficiency of the plant was below the desired levels in terms of COD and BOD. Also the plant operators were not maintaining consistent sludge retention time (SRT). Abrupt discharge of MLSS through the Surplus Activated sludge (SAS) pump was the main reason for the low MLSS in the aeration tank and consequently low treatment performance. In this study the SRT was optimized based on desired MLSS concentration between 3,000–3,500 mg/L and required performance in terms of BOD, COD and TSS. This study revealed that SRT is a very important operational parameter and its knowledge and correct implementation by the plant operators should be mandatory.

Start-up of decentralized MBRs: Part I: The influence of operational parameters

This paper aims to be a quick reference guide to start-up decentralized membrane bioreactors (MBRs). The first part of this study focuses on the impact of different operational parameters on the start-up of decentralized MBRs, which can be easily reproduced in the field. Whereas wastewater is not an option to start-up decentralized MBRs, domestic activated sludge has shown to handle the input of wastewater in a better way than the municipal one. Starting-up at low mixed liquor suspended solids (MLSS) concentration is feasible, and a possible optimum concentration (~ 1 g L − 1 ) has been suggested. In turn, particle size distribution has shown how determined conditions release fine particles in the sub-micron range (0.1–1 μm), impacting negatively the filterability of the initial inoculum and thus the operation. However, in the case of the air scouring rate, even releasing sub-micron particles to the media, high rates demonstrated to extend the operation. Regarding ambient conditions, low temperatures and associated deflocculation processes should be avoided. Chemical oxygen demand and NH 4 +–N removal efficiencies showed values over 87% and 75% respectively whereas suspended solids and removal of pathogens maintained low values (50 mg L − 1 and absence respectively) in the permeate, allowing the reuse of regenerated water since the first day of operation under the different conditions imposed. None of the analyzed parameters (i.e., MLSS, sludge volumetric index and dissolved organic carbon), influenced significantly the filterability of the initial inoculum. Contrarily, the input of wastewater has demonstrated to be the most important factor governing the fouling process of the membrane rather than the changes in the microbiology. As a final consideration, an efficient pretreatment and both low hydraulic retention time and fluxes can help to extend the operation and reach an easy transition between the start-up and the steady-state.

Influence of organic loading rate on the performance of ultrafiltration and microfiltration membrane bioreactors at high sludge retention time

This paper presents the results of 220 days of two bench scale submerged membrane bioreactor (MBR) experiments using micro (MF-MBR) and ultrafiltration (UF-MBR) flat sheet membranes. Both systems operated with a volumetric loading rate of between 0.4 and 1.3kgCODm−3d−1. Results showed that the growth rate of MLSS increased when the loading rate increased and that results related to removal efficiency rendered values higher than 93% chemical oxygen demand (COD) removal in both MBRs, indicating their suitability for operating at different loading rates. As regards membrane fouling, it was observed that the total number of chemical cleanings performed in the two MBRs was similar. The cleaning frequency decreased when the concentration of mixed liquor suspended solids (MLSS) and organic loading increased in both MBRs. MF membranes worked better at extended periods of operation and high concentrations of MLSS, whereas the UF-MBR maintained lower permeability at the beginning of the experiments at MLSS concentrations lower than 2–3g/L. Dead-end filtration experiments showed that the highest filtration resistance was found at the highest soluble extracellular polymeric substance (EPS) concentrations, that is, at low sludge ages or low MLSS concentrations, indicating that membrane fouling is attributed to a large extent to the existence of this kind of EPS.

Fate and removal of permethrin by conventional activated sludge treatment

The fate and removal of permethrin during conventional wastewater treatment were evaluated at pilot‐plant scale at different concentrations of mixed liquor suspended solids (MLSS) and, hence, different solids retention times (SRT). At feed concentrations of 0.26–0.86 µg L−1, the permethrin was removed by primary treatment at an efficiency rate of 37%, similar to previously reported data, and from 40% to 83% for secondary treatment, decreasing with decreasing SRT. Comparable ranges, from 37% up to 98%, have been reported for micropollutants with similar physicochemical properties to permethrin, such as galaxolide and tonalide. Little difference in removal was noted between the medium and low MLSS concentrations trials, the main difference in treated effluent permethrin concentration arising on changing from high to medium MLSS levels. This was attributed to the limited acclimatization period employed in these two trials, leading to higher levels of soluble organic matter in the treated water, with which the permethrin appeared to be associated.

Impact of membrane configuration on fouling in anaerobic membrane bioreactors

The filtration performance of flocculated and granulated configured anaerobic membrane bioreactors (MBR) treating domestic wastewater has been evaluated and compared to conventional aerobic MBR. Immersed hollow fibre (HF) and external tubular membrane geometries were additionally compared with the latter operated in both pumped and gas-lift mode. After 200 d of operation, both granular and flocculated anaerobic MBR (AnMBR) suspensions were characterised by an increased population of colloidal particles while the aerobic MBR retained a unimodal particle size distribution with a d 50 of 20 μm. Consequently, the flocculated AnMBR supernatant was characterised by a soluble microbial product (SMP) concentration ca. 500% higher than the aerobic MBR, such that the lowest critical fluxes for both HF and tubular membranes were recorded for the AnMBR. In comparison, the granulated AnMBR sludge was characterised by a low mixed liquor suspended solids concentration and an SMP concentration below 50% that of the flocculated anaerobic MBR. Consequently, similar fluxes to those of the aerobic MBR were achieved with the granulated anaerobic sludge using immersed HF membranes. Operating external tubular membranes in gas-lift appeared less effective for the granular AnMBR than the Aerobic MBR. However, critical fluxes >40 L m −2 h −1 were achieved using pumped mode. Results suggest granular AnMBR systems to be most suited to domestic wastewater treatment using either immersed HF membranes or external tubular membranes in pumped crossflow mode.

The effect of mixed liquor suspended solids (MLSS) on biofouling in a hybrid membrane bioreactor for the treatment of high concentration organic wastewater

Biofouling is a crucial factor in membrane bioreactor (MBR) applications, particularly for high organic loading operations. This paper reports a study on biofouling in an MBR to establish a relationship between critical flux, Jc, mixed liquor suspended solids (MLSS) (ranging from 5 to 20 g L-1) and volumetric loading rate (6.3 kg COD m-3 h-1) of palm oil mill effluent (POME). A lab-scale 100 L hybrid MBR consisting of anaerobic, anoxic, and aerobic reactors was used with flat sheet microfiltration (MF) submerged in the aerobic compartment. The food-to-microorganism (F/M) ratio was maintained at 0.18 kg COD kg-1 MLSSd-1. The biofouling tendency of the membrane was obtained based on the flux against the transmembrane pressure (TMP) behaviour. The critical flux is sensitive to the MLSS. At the MLSS 20 g L-1 the critical flux is about four times lower than that for the MLSS concentration of 5 g L-1. The results showed high removal efficiency of denitrification and nitrification up to 97% at the MLSS concentration 20 g L-1. The results show that the operation has to compromise between a high and a low MLSS concentration. The former will favour a higher removal rate, while the latter will favour a higher critical flux.

Wastewater treatment by high-growth bioreactor integrated with settling-cum-membrane separation

A different approach of wastewater treatment has been investigated in this study by operating bioreactor at low mixed liquor suspended solids (MLSS) concentrations, thereby maintaining high F/ M ratio to target higher nutrient removal through higher biomass yield. A 10-L capacity laboratory-scale bioreactor followed by settling-cum-membrane separation employing hollow fiber membrane module (pore size: 0.1 μm) was set up and operated under batch mode at a hydraulic retention time (HRT) of 24 h using simulated domestic wastewater as feed. Average removal efficiencies of bioreactor were approximately 84% soluble chemical oxygen demand (SCOD), 95.5% total COD (TCOD), 90% NH 4 +–N, and 87% total Kjeldahl nitrogen (TKN). True yield coefficient ( Y T ) and decay coefficient ( K d ) for bioreactor were estimated at 0.397 kg VSS/kg SCOD and 0.0549 day − 1 , respectively. Significant proliferation of non-flocculating microorganisms was observed in the system with decreasing solids retention time (SRT) and thereby deteriorating biomass settling property indicated by enhanced sludge volume index (SVI). Significant proliferation of non-flocculating microorganisms with better ability to access substrate from bulk solution through smaller mass transfer resistance most probably contributed to better organics removal. The modified approach of wastewater treatment can be a good basis to develop high-growth membrane bioreactor (MBR) as opposed to conventional MBR operation.

Upgrading of Existing Municipal Wastewater Treatment Plants with the “Non-Conventional Low MLSS MBR Technology”

In the future, upgrading of existing wastewater treatment plants (wwtps) will be more important than the erection of entirely new plants, as most of the plants necessary in Germany already exist. Thus far, membrane bioreactor (MBR) technology is not used as an alternative for plant upgrading in Germany. However, at several locations the preconditions indicate that the application of the MBR technology for upgrading of plants may be favorable. These preconditions include the need for a substantial enlargement of the aeration tank volume and/or the need to improve the efficiency of the final sedimentation tanks, and also require that the existing tanks be in a good state, so that the tanks can be used in the future. The relevant preconditions and the basic upgrading concept using the MBR technology were presented earlier. Depending on the local preconditions regarding the existing tank volume, a specific aspect of this application can be an operation mode using non-conventional low MLSS (mixed liquor suspended solids) concentrations (5 kg MLSS/m 3 through 7 kg MLSS/m 3 ) due to the amount of the existing tank volume. Two research projects were carried out covering the operation of five pilot plants on a cubic meter scale to demonstrate the feasibility of this kind of MBR process. This paper presents the core results of this research work.

Specific Resistance to Filtration of Biomass from Membrane Bioreactor Reactor and Activated Sludge: Effects of Exocellular Polymeric Substances and Dispersed Microorganisms

This study investigates the effect of dispersed microorganisms and exocellular polymeric substances on biomass dewaterability. Specific resistance to filtration (SRF) was measured for biomass from a membrane bioreactor and a completely mixed activated sludge system. Both laboratory-scale reactors were fed with synthetic wastewater and operated at a high food-to-microorganism ratio (F/M) (1 to 11 kilograms chemical oxygen demand per kilogram mixed liquor volatile suspended solids per day [kgCOD/(kg MLVSS.d)]) and short solids retention times (0.25 to 5 d). The SRF values were affected by strong interactions of three parameters: (1) the mixed liquor suspended solids concentration, (2) the amount of dispersed microorganisms, and (3) the exocellular polymeric substances (EPS) concentration. At F/M smaller than 2 kg COD/(kg MLSS.d) and mixed liquor suspended solids (MLSS) concentration higher than 2000 mg/L, increasing amount of dispersed microorganisms in the biomass yielded higher SRF values. However, at high F/M (> 5 kg COD/kg MLSS.d) and low MLSS concentrations (< 600 mg/L), lower EPS concentrations resulted in slightly smaller SRF values, even though the amount of dispersed microorganisms in the biomass was much higher. Thus, at low MLSS concentrations, EPS concentrations rather than the amount of dispersed microorganisms tend to control SRF.

Impact of aeration, solids concentration and membrane characteristics on the hydraulic performance of a membrane bioreactor

The concept of critical flux ( J c) has been recognised as a convenient parameter for characterising fouling in membrane bioreactors (MBR), although it has recently been shown that membrane fouling takes place in this specific process under sub-critical operating conditions. It is also generally understood that the permeability of membranes used for biomass separation membrane bioreactors is influenced by the method and rate of aeration, the nature and concentration of the sludge and the membrane characteristics. In this study, J c values have been determined using the flux-step method for various operating conditions of membrane pore and lumen size, mixed liquor suspended solids (MLSS) concentration and aeration rates, and sub-critical operation has also been investigated. Results from factorial experimental design analysis have revealed that (1) the impact of membrane pore size on J c is observed only at low pore size and/or low MLSS levels, (2) no significant difference is apparent for a shift from 4 to 8 gMLSS l −1, but a significant increase of J c arises for an MLSS increase to 12 g l −1, and (3) the MLSS effect on J c was generally around double that of the aeration effect. The calculation of mean sub-critical values for a range of pressure-related critical parameters revealed reduced short-term fouling at larger-pore sizes, but that this may be coupled with internal membrane fouling.