Influence of environmental conditions on accumulated polyhydroxybutyrate in municipal activated sludge

Poly(3-hydroxybutyrate) (PHB) was accumulated in full-scale municipal waste activated sludge at pilot scale. After accumulation, the fate of the PHB-rich biomass was evaluated over two weeks as a function of initial pH (5.5, 7.0 and 10), and incubation temperature (25, 37 and 55°C), with or without aeration. PHB became consumed under aerobic conditions as expected with first order rate constants in the range of 0.19 to 0.55 d^-1. Under anaerobic conditions, up to 63 percent of the PHB became consumed within the first day (initial pH 7, 55°C). Subsequently, with continued anaerobic conditions, the polymer content remained stable in the biomass. Degradation rates were lower for acidic anaerobic incubation conditions at a lower temperature (25°C). Polymer thermal properties were measured in the dried PHB-rich biomass and for the polymer recovered by solvent extraction using dimethyl carbonate. PHB quality changes in dried biomass, indicated by differences in polymer melt enthalpy, correlated to differences in the extent of PHB extractability. Differences in the expressed PHB-in-biomass melt enthalpy that correlated to the polymer extractability suggested that yields of polymer recovery by extraction can be influenced by the state or quality of the polymer generated during downstream processing. Different post-accumulation process biomass management environments were found to influence the polymer quality and can also influence the extraction of non-polymer biomass. An acidic post-accumulation environment resulted in higher melt enthalpies in the biomass and, consequently, higher extraction efficiencies. Overall, acidic environmental conditions were found to be favourable for preserving both quantity and quality after PHB accumulation in activated sludge.

Ionic strength of the liquid phase of different sludge streams in a wastewater treatment plant

In a wastewater treatment plant (WWTP), several sludge streams exist and the composition of their liquid phase varies with time and place. For evaluating the potential for formation of precipitates and equilibria for weak acids/bases, the ionic strength and chemical composition need to be known. This information is often not available in literature, and even neglected in chemical model-based research. Based on a literature review, we proposed three ranges of concentration (low, typical and high) for the major constituents of the liquid phase of the different streams in a WWTP. The study also discusses the reasons for the concentration evolution, and the exceptional cases, to allow readers to consider the right range depending on their situation. The ionic strength of the different streams and the contribution of its constituents were calculated based on the ionic composition. The major contributors to the ionic strength for the wastewater-based streams (influent, effluent and mixed sludge) were Na⁺, Cl^-, Mg²⁺ and Ca²⁺, representing 50-70% of the ionic strength. For digestate, NH₄⁺ and HCO₃^- accounted for 65-75% of the ionic strength. Even though the ionic strength is recognized to impact several important wastewater treatment processes, its utilization in literature is not always adequate, which is discussed in this study.

Full-scale increased iron dosage to stimulate the formation of vivianite and its recovery from digested sewage sludge

The recovery of phosphorus from secondary sources like sewage sludge is essential in a world suffering from resources depletion. Recent studies have demonstrated that phosphorus can be magnetically recovered as vivianite (Fe(II)₃(PO₄)₂∗8H₂O) from the digested sludge (DS) of Waste Water Treatment Plants (WWTP) dosing iron. To study the production of vivianite in digested sludge, the quantity of Fe dosed at the WWTP of Nieuwveer (The Netherlands) was increased (from 0.83 to 1.53 kg Fe/kg P in the influent), and the possible benefits for the functioning of the WWTP were evaluated. Higher Fe dosing is not only relevant for P-recovery, but also for maximal recovery of organics from influent for e.g. biogas production. The share of phosphorus present as vivianite in the DS increased from 20% to 50% after the increase in Fe dosing, making more phosphorus available for future magnetic recovery. This increase was directly proportional to the increase of Fe in DS, suggesting that vivianite could be favored not only thermodynamically, but also kinetically. Interestingly, analyses suggest that several types of vivianite are formed in the WWTP, and could differ in their purity, oxidation state or crystallinity. These differences could have an impact on the subsequent magnetic separation. Following the Fe dosing increase, P in the effluent and H₂S in the biogas both decreased: 1.28 to 0.42 ppm for P and 26 to 8 ppm for H₂S. No negative impact on the nitrogen removal, biogas production, COD removal or dewaterability was observed. Since quantification of vivianite in DS is complicated, previous studies were reviewed and we proposed a more accurate Mössbauer spectroscopy analysis and fitting for sludge samples. This study is important from a P recovery point of view, but also because iron addition can play a crucial role in future resource recovery wastewater facilities.

Role of feed water biodegradable substrate concentration on biofouling: Biofilm characteristics, membrane performance and cleanability

Biofouling severely impacts operational performance of membrane systems increasing the cost of water production. Understanding the effect of critical parameters of feed water such as biodegradable substrate concentration on the developed biofilm characteristics enables development of more effective biofouling control strategies. In this study, the effect of substrate concentration on the biofilm characteristics was examined using membrane fouling simulators (MFSs). A feed channel pressure drop (PD) increase of 200 mbar was used as a benchmark to study the developed biofilm. The amount and characteristics of the formed biofilm were analysed in relation to membrane performance indicators: feed channel pressure drop and permeate flux. The effect of the characteristics of the biofilm developed at three substrate concentrations on the removal efficiency of the different biofilms was evaluated applying acid/base cleaning. Results showed that a higher feed water substrate concentration caused a higher biomass amount, a faster PD increase, but a lower permeate flux decline. The permeate flux decline was affected by the spatial location and the physical characteristics of the biofilm rather than the total amount of biofilm. The slower growing biofilm developed at the lowest substrate concentration was harder to remove by NaOH/HCl cleanings than the biofilm developed at the higher substrate concentrations. Effective biofilm removal is essential to prevent a fast biofilm regrowth after cleaning. While substrate limitation is a generally accepted biofouling control strategy delaying biofouling, development of advanced cleaning methods to remove biofilms formed under substrate limited conditions is of paramount importance.

Vivianite as the main phosphate mineral in digested sewage sludge and its role for phosphate recovery

Phosphate recovery from sewage sludge is essential in a circular economy. Currently, the main focus in centralized municipal wastewater treatment plants (MWTPs) lies on struvite recovery routes, land application of sludge or on technologies that rely on sludge incineration. These routes have several disadvantages. Our study shows that the mineral vivianite, Fe₂(PO₄)₃ × 8H₂O, is present in digested sludge and can be the major form of phosphate in the sludge. Thus, we suggest vivianite can be the nucleus for alternative phosphate recovery options. Excess and digested sewage sludge was sampled from full-scale MWTPs and analysed using x-ray diffraction (XRD), conventional scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX), environmental SEM-EDX (eSEM-EDX) and Mössbauer spectroscopy. Vivianite was observed in all plants where iron was used for phosphate removal. In excess sludge before the anaerobic digestion, ferrous iron dominated the iron pool (≥50%) as shown by Mössbauer spectroscopy. XRD and Mössbauer spectroscopy showed no clear correlation between vivianite bound phosphate versus the iron content in excess sludge. In digested sludge, ferrous iron was the dominant iron form (>85%). Phosphate bound in vivianite increased with the iron content of the digested sludge but levelled off at high iron levels. 70-90% of all phosphate was bound in vivianite in the sludge with the highest iron content (molar Fe:P = 2.5). The quantification of vivianite was difficult and bears some uncertainty probably because of the presence of impure vivianite as indicated by SEM-EDX. eSEM-EDX indicates that the vivianite occurs as relatively small (20-100 μm) but free particles. We envisage very efficient phosphate recovery technologies that separate these particles based on their magnetic properties from the complex sludge matrix.

Vivianite as an important iron phosphate precipitate in sewage treatment plants

Iron is an important element for modern sewage treatment, inter alia to remove phosphorus from sewage. However, phosphorus recovery from iron phosphorus containing sewage sludge, without incineration, is not yet economical. We believe, increasing the knowledge about iron-phosphorus speciation in sewage sludge can help to identify new routes for phosphorus recovery. Surplus and digested sludge of two sewage treatment plants was investigated. The plants relied either solely on iron based phosphorus removal or on biological phosphorus removal supported by iron dosing. Mössbauer spectroscopy showed that vivianite and pyrite were the dominating iron compounds in the surplus and anaerobically digested sludge solids in both plants. Mössbauer spectroscopy and XRD suggested that vivianite bound phosphorus made up between 10 and 30% (in the plant relying mainly on biological removal) and between 40 and 50% of total phosphorus (in the plant that relies on iron based phosphorus removal). Furthermore, Mössbauer spectroscopy indicated that none of the samples contained a significant amount of Fe(III), even though aerated treatment stages existed and although besides Fe(II) also Fe(III) was dosed. We hypothesize that chemical/microbial Fe(III) reduction in the treatment lines is relatively quick and triggers vivianite formation. Once formed, vivianite may endure oxygenated treatment zones due to slow oxidation kinetics and due to oxygen diffusion limitations into sludge flocs. These results indicate that vivianite is the major iron phosphorus compound in sewage treatment plants with moderate iron dosing. We hypothesize that vivianite is dominating in most plants where iron is dosed for phosphorus removal which could offer new routes for phosphorus recovery.

Effect of water temperature on biofouling development in reverse osmosis membrane systems

Understanding the factors that determine the spatial and temporal biofilm development is a key to formulate effective control strategies in reverse osmosis membrane systems for desalination and wastewater reuse. In this study, biofilm development was investigated at different water temperatures (10, 20, and 30 °C) inside a membrane fouling simulator (MFS) flow cell. The MFS studies were done at the same crossflow velocity with the same type of membrane and spacer materials, and the same feed water type and nutrient concentration, differing only in water temperature. Spatially resolved biofilm parameters such as oxygen decrease rate, biovolume, biofilm spatial distribution, thickness and composition were measured using in-situ imaging techniques. Pressure drop (PD) increase in time was used as a benchmark as to when to stop the experiments. Biofilm measurements were performed daily, and experiments were stopped once the average PD increased to 40 mbar/cm. The results of the biofouling study showed that with increasing feed water temperature (i) the biofilm activity developed faster, (ii) the pressure drop increased faster, while (iii) the biofilm thickness decreased. At an average pressure drop increase of 40 mbar/cm over the MFS for the different feed water temperatures, different biofilm activities, structures, and quantities were found, indicating that diagnosis of biofouling of membranes operated at different or varying (seasonal) feed water temperatures may be challenging. Membrane installations with a high temperature feed water are more susceptible to biofouling than installations fed with low temperature feed water.

A novel scenario for biofouling control of spiral wound membrane systems

Current strategies to control biofouling in nanofiltration and reverse osmosis membrane systems such as chemical cleaning and use of low fouling membranes are not always successful. Based on recent studies, an alternative approach is derived, combining a lower linear flow velocity in lead modules and adapted designs for feed spacer with an advanced cleaning strategy. This approach can be realized by small adaptations in current plant design. A lower linear flow velocity in lead spiral wound membrane modules results in (i) lower energy use, (ii) lower impact of biomass on the feed channel pressure drop, and (iii) more fluffy biofilm that may be easier to remove from the lead membrane modules, especially when adapted feed spacers combined with a reversed enhanced flush are applied. This rational scenario can result in effective biofouling control at low energy requirements, minimal chemical use and minimal cost.

Phosphate limitation to control biofouling

Phosphate limitation as a method to control biofouling of spiral wound reverse osmosis (RO) membranes was studied at a full-scale installation fed with extensively pretreated water. The RO installation is characterized by (i) a low feed channel pressure drop increase and (ii) low biomass concentrations in membrane elements at the installation feed side. This installation contrasted sharply with installations fed with less extensively pretreated feed water (and therefore higher phosphate concentrations) experiencing a high-pressure drop increase and high biomass concentrations in lead elements. Membrane fouling simulator (MFS) studies showed that low phosphate concentrations (approximately 0.3 microg P L(-1)) in the feed water restricted the pressure drop increase and biomass accumulation, even at high substrate (organic carbon) concentrations. In the MFS under ortho-phosphate limiting conditions, dosing phosphonate based antiscalants caused biofouling while no biofouling was observed when acids or phosphonate-free antiscalants were used. Antiscalant dosage could increase both phosphate and substrate concentrations of the water. Therefore, antiscalant selection may be critical for biofouling control. Since no biofouling was observed at low phosphate concentrations, restricting biomass growth by phosphate limitation may be a feasible approach to control biofouling, even in the presence of high organic carbon levels.

Integrated approach for biofouling control

Despite extensive research efforts, past and present strategies to control biofouling problems in spiral-wound nanofiltration and reverse osmosis membranes have not been successful under all circumstances. Gaining insight in the biofouling process is a first necessity. Based on recent insights, an overview is given of 12 potential complementary approaches to solve biofouling. Combinations of approaches may be more efficient in biofouling control than a single approach. A single approach must be 100% effective, while in combination each individual approach can be partially effective while the combination is still efficient. An integrated Approach for Biofouling Control (ABC) is proposed, based on three corner stones: (i) equipment design and operation, (ii) biomass growth conditions, and (iii) cleaning agents as a framework to control biofouling. While past and present strategies addressed mainly membranes and microorganisms, i.e. removal or inactivation of biomass, this ABC-approach addresses the total membrane filtration system. It is anticipated that this integral approach will enable a more rational and effective control of biofouling. Although in this stage chemical cleaning and biofouling inhibitor dosage seem unavoidable to control biofouling, it is expected that in future--because of sustainability and costs reasons--membrane systems will be developed without or with minimal need for chemical cleaning and dosing. Three potential scenarios for biofouling control are proposed based on (i) biofouling tolerant spiral wound membrane systems, (ii) capillary membranes, and (iii) phosphate limitation.

Pressure drop increase by biofilm accumulation in spiral wound RO and NF membrane systems: role of substrate concentration, flow velocity, substrate load and flow direction

In an earlier study, it was shown that biofouling predominantly is a feed spacer channel problem. In this article, pressure drop development and biofilm accumulation in membrane fouling simulators have been studied without permeate production as a function of the process parameters substrate concentration, linear flow velocity, substrate load and flow direction. At the applied substrate concentration range, 100-400 microg l(-1) as acetate carbon, a higher concentration caused a faster and greater pressure drop increase and a greater accumulation of biomass. Within the range of linear flow velocities as applied in practice, a higher linear flow velocity resulted in a higher initial pressure drop in addition to a more rapid and greater pressure drop increase and biomass accumulation. Reduction of the linear flow velocity resulted in an instantaneous reduction of the pressure drop caused by the accumulated biomass, without changing the biofilm concentration. A higher substrate load (product of substrate concentration and flow velocity) was related to biomass accumulation. The effect of the same amount of accumulated biomass on the pressure drop increase was related to the linear flow velocity. A decrease of substrate load caused a gradual decline in time of both biomass concentration and pressure drop increase. It was concluded that the pressure drop increase over spiral wound reverse osmosis (RO) and nanofiltration (NF) membrane systems can be reduced by lowering both substrate load and linear flow velocity. There is a need for RO and NF systems with a low pressure drop increase irrespective of the biomass formation. Current efforts to control biofouling of spiral wound membranes focus in addition to pretreatment on membrane improvement. According to these authors, adaptation of the hydrodynamics, spacers and pressure vessel configuration offer promising alternatives. Additional approaches may be replacing heavily biofouled elements and flow direction reversal.

Modelling of an oil refinery wastewater treatment plant

The Activated Sludge Model No. 3 (ASM3) and Dutch calibration guidelines (STOWA) were evaluated in the modelling of an activated sludge system treating effluents from a large oil refinery. The plant was designed to remove suspended solids, organic matter and nitrogen from wastewater at an average water temperature of 34 degrees C. The plant consists of three tanks in series; the first two tanks operate in on-off aeration mode with pure oxygen for N-removal, whilst extra methanol is added for the denitrification, and the third tank is maintained as constantly aerobic. Calibration was performed based on a simplified influent characterisation and extra batch experiments (nitrification and denitrification). With the adjustment of only four parameters the model proved capable of describing the performance of the plant concerning both the liquid phase and the biomass. The model was further used to analyse possible modifications in the plant layout and optimize operational conditions in order to reduce operating costs. Modelling results indicated reduction in methanol dosage by implementing an idle time between aerobic and anoxic phases. In this way, surplus methanol was prevented from entering during the aerobic period. Moreover, simulations showed that the most cost-effective option regarding the denitrification process was a combined pre-post-denitrification scheme, without the need for enlarging existing basins. It can be concluded that although ASM3 and STOWA guidelines were originally developed for domestic wastewater application at a temperature range of 10 to 20 degrees C, they proved well capable of describing the performance of an oil refinery wastewater treatment plant operating at 34 degrees C. Moreover, the plant model proved useful for optimization of the plant performance regarding operational costs.

Investigation of microbial communities on reverse osmosis membranes used for process water production

In the present study, the diversity and the phylogenetic affiliation of bacteria in a biofouling layer on reverse osmosis (RO) membranes were determined. Fresh surface water was used as a feed in a membrane-based water purification process. Total DNA was extracted from attached cells from feed spacer, RO membrane and product spacer. Universal primers were used to amplify the bacterial 16S rRNA genes. The biofilm community was analysed by 16S rRNA-gene-targeted denaturing gradient gel electrophoresis (DGGE) and the phylogenetic affiliation was determined by sequence analyses of individual 16S rDNA clones. Using this approach, we found that five distinct bacterial genotypes (Sphingomonas, Beta proteobacterium, Flavobacterium, Nitrosomonas and Sphingobacterium) were dominant genera on surfaces of fouled RO membranes. Moreover, the finding that all five "key players" could be recovered from the cartridge filters of this RO system, which cartridge filters are positioned before the RO membrane, together with literature information where these bacteria are normally encountered, suggests that these microorganisms originate from the feed water rather than from the RO system itself, and represent the fresh water bacteria present in the feed water, despite the fact that the feed water passes an ultrafiltration (UF) membrane (pore size approximately 40 nm), which is able to remove microorganisms to a large extent.

Waste characterization for implementation in ADM1

Wastewater characterization as required for implementation in ADM1 is based on the identification of the numerous concentrations of the specific compounds defined in ADM1. However, identification of the individual substrate concentrations requires specific analytical techniques and in most cases only general measurements like COD, TOC, and organic nitrogen are available. This paper describes a simple method for calculation of the lumped elemental composition of the organic substrates in the wastewater from a limited number of widely available analyses. Using the elemental composition of the lumped substrate and the elemental composition of the substrates defined in the model, the influent composition as required for input in ADM1 can be calculated. Furthermore, proper waste characterization allows for an initial analysis of the biogas flow rate and composition as well as the reactor pH that can be achieved upon organic substrate degradation, as will be demonstrated. It is hoped that the methods described in this paper will stimulate and simplify future application of ADM1.

Production of polyhydroxyalkanoates by mixed culture: recent trends and biotechnological importance

Polyhydroxyalkanoates (PHAs) are the polymers of hydroxyalkanoates that accumulate as carbon/energy or reducing-power storage material in various microorganisms. PHAs have been attracting considerable attention as biodegradable substitutes for conventional polymers. To reduce their production cost, a great deal of effort has been devoted to developing better bacterial strains and more efficient fermentation/recovery processes. The use of mixed cultures and cheap substrates can reduce the production cost of PHA. Accumulation of PHA by mixed cultures occurs under transient conditions mainly caused by intermittent feeding and variation in the electron donor/acceptor presence. The maximum capacity for PHA storage and the PHA production rate are dependent on the substrate and the operating conditions used. This work reviews the development of PHA research. Aspects discussed include metabolism and various mechanisms for PHA production by mixed cultures; kinetics of PHA accumulation and conversion; effects of carbon source and temperature on PHA production using mixed cultures; PHA production process design; and characteristics of PHA produced by mixed cultures.

Production of polyhydroxyalkanoates by mixed microbial cultures

Polyhydroxyalkanoates (PHAs) are biodegradable bioplastics formed from renewable resources, like sugars, with similar characteristics of polypropylene. These bioplastics are industrially produced by pure cultures using expensive pure substrates. These factors lead to a much higher selling price of PHAs compared to petroleum-based plastics, like polypropylene. The use of mixed cultures and cheap substrates (waste materials) can reduce costs of PHA production by more than 50%. Storage of PHAs by mixed populations occurs under transient conditions mainly caused by discontinuous feeding and variation in the electron donor/acceptor presence. In the last years the mechanisms of storage, metabolism and kinetics of mixed cultures have been studied. The maximum capacity of PHA storage and production rate is dependent on the substrate and on the operating conditions used. In this paper an overview and discussion of various mechanisms and processes for PHA production by mixed cultures is presented.

Bio-augmentation by nitrification with return sludge

Bio-augmentation can be used to obtain nitrification in activated sludge processes that operate at sub-optimal solid retention times. In this study, we evaluated the potential of augmenting the endogenous nitrifying bacteria, by implementing a nitrification reactor in the sludge return line. This reactor can be fed with an internal N-rich flow (e.g. effluent from the sludge treatment) or with an external ammonium source. A mathematical model based on ASM1 was developed and used to evaluate the potential of this technique. The bio-augmentation studied here aimed to enhance the nitrification process of highly loaded activated sludge systems. A calibrated simulation model of a high loaded wastewater treatment plant in The Netherlands was used for this study. A side stream process (the named BABE process) was included in the simulation. This process was fed with the ammonia-rich water generated by sludge digestion and subsequent thickening by centrifugation (the so-called rejectwater). An external source (artificial) of ammonium was also considered to evaluate the differences between the two origins of ammonium. The results showed that with the augmentation process, high loaded activated sludge systems can achieve nitrification even at low winter temperatures. The best effect is obtained for systems operating at approximately 50% of the minimal SRT without augmentation. The use of an internal ammonia source is more effective than an external source. The results of this study give a quantitative basis for the design of process internal bio-augmentation processes and the effect on the N-removal capacity of the treatment plant.

Poly-beta-hydroxybutyrate metabolism in dynamically fed mixed microbial cultures

The kinetics of production and degradation of poly-beta-hydroxybutyrate (PHB) by a mixed activated sludge culture growing on acetate was studied in a sequencing batch reactor (SBR). Occasionally a very high amount of acetate was added to the steady state system in order to obtain high PHB concentrations in the cells (fPHB). This made it possible to follow PHB production and degradation over a wide range of fPHB-data (between 0 and 0.8 Cmol/Cmol). The results were compared with data available in literature and with equations derived by metabolic modeling. This led to some remarkable observations. For the feast period, the ratio q(feast)PHB/-q(feast)Ac (specific PHB production rate over specific acetate uptake rate) was used to indicate which fraction of the substrate is stored. Experimentally and theoretically it was shown that this ratio has a constant value for dynamically fed systems operated at a sludge retention time (SRT) > 2d. This value is 0.6 Cmol/Cmol under aerobic conditions and 0.4-0.5 Cmol/Cmol under anoxic conditions, irrespective of the specific growth rate of the biomass and the specific acetate uptake rate in the feast period. Degradation of internal stored PHB could be described with a first order degradation rate with respect to the PHB content of the cells. Degradation of PHB appeared to be independent of the type of electron acceptor present in the system and independent of the SRT of the system. The kinetic descriptions can be used to predict PHB production and consumption in general in dynamic fed wastewater treatment systems, and they provide some trends for modeling purposes.