Membrane Enhanced Biological Phosphorous Removal CE-482 Self Study Presentation By :Srinivasa Nookala
WHY?•An increase might change water quality or food webs.•Excess algae: scum, noxious blue-greens, taste/odor/smell•O2 depletion; loss of fish habitat•Loss of clarity; aesthetic loss•Excess macrophyte (“weed”) growth- loss of open water;•Favors exotic species (EWM); sediment destabilization•Lower bottom O2: increased sediment nutrient release: loss of fish habitat•Loss of native macrophytes from algal shading; loss of fish & water fowl habitat and food; reduced shoreline & bottom•Stabilization, increased erosion•Excess organic matter: smothers eggs and bugs
SO??• Stringent measure have to be taken to counter these.• This calls for advanced treatment methods which are also cost effective which can help us conform with the strict norms.
Existing technologiesPhysical:1. Filtration for particulate phosphorus2. Membrane technologiesChemical:1. Precipitation2. Other (mainly physical-chemical adsorption)Biological1. Assimilation2. Enhanced biological phosphorus removal (EBPR)The greatest interest and most recent progress has been made in EBPR,which has the potential to remove P down to very low levels at relativelylower costs. Membrane technologies are also receiving increasedattention, although their use for P removal has been more limited to date.
Some problemsPhysicochemical Precipitation• 25% sludge production increase• High level of chemical consumption resulting in high costs.• Salinity increase of the effluent• Potential detrimental impact on the biological nitrification, due to resulting low alkalinity and extreme pH.•High level of metallic impurities.
Why not just bio process??• Build up of phosphates in the system.• The recycled activated sludge is treated which results in release of phosphorus taken up during the process.• This results in reduced efficiency and increased phosphorus content in the effluent.• Limitations imposed by settling characteristics of sludge in clarifier (high process solid levels or high SRT not possible).
What is membrane enhanced….??3 zones1. Anaerobic zone - an anaerobic mixed liquor has organisms which release phosphorous into the anaerobic mixed liquor and store volatile fatty acids from the anaerobic mixed liquor.2. Anoxic zone - an anoxic mixed liquor has organisms which metabolize stored volatile fatty acids, uptake phosphorous and denitrify the anoxic mixed liquor.3. Aerobic zone - an aerobic mixed liquor has organisms which metabolize stored volatile fatty acids, uptake phosphorous and nitrify the aerobic mixed liquor.
Schematic diagram of the process Influent water anaerobic anoxic membrane aerobic Effluent water reject
ProcessesAnaerobic site - fermentive bacteria convert BOD into volatile fatty acids. Bio-Porganisms use the volatile fatty acids as a carbon source. In doing so, theyrelease phosphorus into the liquor, and store volatile fatty acids.In the anoxic and aerobic zones, the Bio-P organisms metabolize the storedvolatile fatty acids and uptake phosphates from the liquor. The recyclebetween the anoxic and anaerobic zones allows the process to operatesubstantially continuously.The stream exiting the aerobic zone passes through the membrane filter. In themembrane filter, phosphorus-rich activated sludge, finely suspended colloidalphosphorus, bacteria, and other cellular debris are rejected by the membrane.A phosphorous lean effluent is produced at the permeate side of themembrane filter. The effluent is also reduced in nitrogen as a result ofthe anoxic and aerobic zones and the recycle between them.
Processes Phosphorus uptake Influent water Phosphorus released Denitrification of mixed liquor membrane Phosphorus uptake Denitrification of mixed liquor Effluent water reject
How better??Membrane Bio Reactors-Advantages of membrane filtration• Complete solid-liquid separation• Prevents failure of biological systems due to biomass loss or bulking• Maintains high Mixed Liquor Suspended Solids (MLSS) in the reactor.• In addition to removing the P in the TSS, membranes also can remove dissolved P. Membrane bioreactors (MBRs, which incorporate membrane technology in a suspended growth secondary treatment process), tertiary membrane filtration (after secondary treatment), and reverse osmosis (RO) systems have all been used in full-scale plants with good results.
Advantages- EBPR•Enhanced Biological Phosphorous removal (EBPR) process is cost-effectivethan co-precipitation.•This comparison is favored by high phosphorus content in the raw water(disadvantage for co-precipitation as much coagulant is required to achievethe discharge criteria), or by high COD/P ratio (advantage for EBPR as morephosphorus can be biosorbed).
Until recently not much effort was made to adapt EBPR processes to the MBR technology.• Given the high solid retention times (SRTs) of the early MBR systems –up to 50 days–, it was considered that EBPR could not be efficient and cost-effective compared to co-precipitation.• Recent observations indicate that efficient and stable phosphorus removal could be set up in MBR systems with high SRT due to the complete retention of solids and biomass through the membrane, the absence of sludge bulking or flush-out, the final aerobic reconditioning of the biomass in the membrane vessel, instead of anoxic or anaerobic conditions in the clarifier.
Two major advantages of MBR on Conventional Wastewater TreatmentPlants (CWWTP) can be noted for EBPR efficiency:Lower effluent phosphorus concentrations are achieved through(i) Complete removal of all particles (containing usually up to 0.1mgP/mgTS), and(ii) final aeration in the membrane tank which allows to prevent from phosphate release during the separation phase (settled sludge in clarifiers is more or less anoxic).This achieves also a final “reconditioning” of the sludge:A complete P-uptake and COD degradation before being returned inthe system.
Advantages continued.. The membrane filter removes colloidal phosphorus and bacteria which would normally pass through a clarifier. Although the absolute amount of colloidal solids is relatively small, the percentage of phosphorus in the colloids is surprisingly high and its removal results in unexpected low levels of phosphorus in the effluent. With membrane filters to remove biomass from the effluent stream, a fine biomass can be maintained in the anaerobic reactor. This may result in enhanced reaction rates and higher than anticipated release of phosphorus in the anaerobic reactor, with resulting higher uptake of phosphorus in the anoxic and aerobic zones. Further, since the process is not limited by the settling characteristics of the sludge, the process is able to operate at very high process solid levels, preferably with an MLSS between 3 and 30 mg/L and short net hydraulic retention times, preferably between 2 and 12 hours. The short HRT allows increased throughput of waste water for a given reactor size.
…..In addition, since the design avoids chemical precipitation of phosphatesupstream of the membrane filters, there is reduced membrane fouling whichfurther enhances the performance of the process. Moreover, contaminants inthe sludge resulting from precipitating chemicals are reduced permitting thesystem to operate at a high sludge age. At high sludge retention times,preferably between 10 and 30 days, an unexpected significant crystallinephosphorus accumulation occurs in the biomass, effectively removingphosphorus from the system. As well, there is lower net sludge generation.
ResultsAn experimental reactor was set up. The membrane filter consisted of fourZEEWEEDTM ZW-10TM modules produced by Zenon Environmental Inc.having a total of 40 square feet of membrane surface area. A control reactorwas set up using a clarifier instead of the membrane filter , recycling theclarifier bottoms to the anoxic zone and not using a retentate recycle streamor nitrified liquor recycle . Both reactors had a volume of 1265 L, the volumeof the clarifier not being counted as reactor volume. Sludge retentiontime (SRT) was kept constant at 25 days.Three experimental runs were conducted with the experimental reactor athydraulic retention times (HRTs) of 9 hours, 6 hours and 4.5 hours producedby varying the feed flow rate. The control reactor was run successfully at ahydraulic retention time of 9 hours using the same operating parameters asfor the run of the experimental reactor with a 9 hour HRT. Running thecontrol reactor at a hydraulic retention time of 6 hours was attempted, butadequate operation could not be achieved (because the clarifier failed), mostconventional processes running at an HRT of about 12. The sizes of the zonesand the HRTs of each zone are summarized in Table 1 below
During the first run, the experimental and control reactors wereoperated at a 9 hour HRT for 16 weeks. The MLSS concentrationvaried between 3-5 g/L during this period. A summary of theaverage P and N concentrations for both reactors is shown inTable. Effluent P was generally below 0.3 mg/L for theexperimental process while effluent P for the control processvaried from 0.2-0.7 mg/L.
During the second run, the experimental reactor was operated at a 6hour HRT for about 14 weeks. The MLSS concentration increased fromabout 4 mg/L at the start to about 8 mg/L at the end of the run. By theend of the run, the experimental process had stabilized in terns of VFAuptake and phosphorous release in the anaerobic section. There was aslow and steady improvement in performance as the experimental runprogressed, the monthly average effluent P dropping from 0.178 mg/Lto 0.144 mg/L to 0.085 mg/L over the approximately three months ofthe test.During the third run, the experimental reactor only was run at an HRTof 4.5 hours. MLSS concentration increased to 15 g/L. Effluent Pconcentrations were generally below 0.5 mg/L over a three monthperiod, still better than the P removal of the control reactor operatedat a 9 hour HRT.
References 1. Aquatic Ecosystem P – Prof. S Mohan 2. Biological process for removing phosphorus involving a membrane filter, United States Patent 6485645 – Husain, Hidayat; Koch, Frederic; Phagoo, Deonarine