Development of a hybrid ozonation bio

Development of a hybrid ozonation biofilm-membrane filatration process for the production of drinking water T.Leiknes,zarova and H.Ødegaard
NTNU-Norwegian University of Science and Technology,Department of Hydraulic and Environmental Engineering,S.P.Andersensvei5,N-7491Trondheim,Norway(E-mail:torove.leiknes@ntnu.no)
Abstract Drinking water sources in Norway are characterized by high concentrations of natural organic matter(NOM),low alkalinity and low turbidity.The removal of NOM is therefore a general requirement in producing potable water.Drinking water treatment plants are commonly designed with coagulation direct filtration or NF spiral wound membrane processes.This study has investigated the feasibility and potential of a hybrid process combining ozonation and biofiltration with a rotating disk membrane for treating drinking water with high NOM concentrations.Ozonation will oxidize the NOM content removing colour and form biodegradable organic compounds,which can be removed in biologicalfilters.A constructed water was used in this study which is representative of ozonated NOM-containing water.A rotating membrane disk bioreactor downstream the ozonation process was used to carry out both the biodegradation as well as biomass separation in the same reactor.Maintenance of biodegradation of the organic matter while controlling biofouling of the membrane and acceptable water production rates was the focus in the study.Three operating modes were investigated.Removal of the biodegradable organics was consistent throughout the study indicating that sufficient biomass was maintained in the reactor for all operating conditions tested.Biofouling control was not achieved through shear-induced cleaning by periodically rotating the membrane disks at high speed.By adding a small amount of sponges in the membrane chamber the biofouling could be controlled by mechanical cleaning of the membrane surface during disk rotation.The overall results indicate that the system can favorably be used in an
ozonation/biofiltration process by carrying out both biodegradation as well as biomass separation in the same reactor.
Keywords Membranefiltration;rotating membrane disk unit;NOM removal;ozonation/bio-membrane
filtration
Introduction
Due to climatic and geographical conditions Norway has an abundance of water resources and about90%of drinking water supplies in Norway are therefore from surface water sources,generally from lakes with very low turbidity.In general,the drinking water sources are characterized by high concentrations of natural organic matter(NOM),low pH,low alkalinity and low turbidity(Ødegaard et al.,1999).Typical values are;colour of30–80mg Pt/l,TOC3–6mg C/l,COD4–8mg Mn/l,turbidity,1NTU,alkalinity ,0.5meq/1and hardness,5mg Ca/l.The removal of NOM is a primary treatment con-cern since coloured water is unattractive to consumers,results in colouring of clothes during washing,can cause odour and taste,increases corrosion and biofilm growth in the distribution network,and is a precursor to the formation of disinfection by-products (DBP)when water is disinfected.Halogenated compounds resulting from chlorination of drinking water containing concentrations of NOM has been a major concern since their discovery in the early1970s.As some of the chlorination by-products are carcinogens many studies and research programs have been undertaken to minimize DBP formation in drinking water.One possibility is to eliminate the chlorination step in water treatment and use alternative disinfection e of ozone as the primary disinfectant is increasing but the formation of ozonation by products is also of concern,particularly due Water Science & Technology Vol 51 No 6–7 pp 241–248 Q IWA Publishing 2005 241
to the increase in biogrowth potential and formation of certain compounds that may be a health concern (Glaze et al.,1989a,b ).The process will oxidize the NOM content remov-ing colour and forming more easily biodegradable organic matter in the water (Carlson and Amy,1997;Melin and Ødegaard,2000).The formation of biodegradable organic carbon in drinking water can cause regrowth problems in the distribution network.Bio-logically active filters can,however,be used to remove the biodegradable fraction from the ozonated water.This is the basis for developing an ozonation/biofiltration process for the removal of NOM in drinking water (Ødegaard,1996;Ødegaard et al.,1999;Melin and degaard,2000).
In previous studies it was found that the ozone dosage is primarily dependent on the final colour aimed for in the water,and a dosage equivalent to 1.0g O 3/g TOC raw water or 0.15mg O 3/mg Pt raw water was necessary (Flgstad and Ødegaard,1985;Melin et al.,2002).These ozone dosages are far higher than those needed for bacteria and virus disin-fection and Giardia inactivation,however,even higher dosages may be needed to elimin-ate Cryptosporidium (Clark et al.,2002).
Kimura (2000)demonstrated that biological nitrification of river water could be obtained by combining biodegradation and biomass separation in a rotating membrane reactor.This lead to experiments in which the potential of a drinking water treatment pro-cess for the removal of NOM based on ozonation followed by a biofilter with membrane separation of the biomass in a rotating membrane reactor was investigated (Ohme,2002;Leiknes et al.,2003).In these experiments a constructed water that is representative for typical Norwegian surface water sources was prepared by adding a NOM concentrate to tap water.The rotating membrane disk unit performed well and the final permeate water quality was well within the requirements of the national drinking water standards.Foul-ing of the membrane and fouling control by varying operating conditions and utilizing the self-cleaning effect of rotating the membrane disks was investigated.A design flux around 201/m 2h was proposed for the unit,however,more efficient fouling control and cleaning of the membranes is necessary to exploit the potential of the process.(Ohme,2002;Leiknes et al.,2003)
The objective of this study has been to further develop the hybrid process combining ozonation and biofiltration with a rotating disk membrane for treating drinking water with high NOM concentrations.In particular,this study has focused on the development and maintenance of the biofilm on the membrane to obtain the necessary biodegradation of the biodegradable NOM while controlling biofouling and maintaining an acceptable water production (i.e.flux).Methods Membrane pilot plant The membrane system used in this study was a Rotary Multi-Disk Type Membrane Separator (RDM)provided by Hitachi Plant Engineering &Construction Co.,Ltd.,Japan.The system is based on flat sheet membranes attached to rotary disks positioned on shafts.The permeate is extracted through the shaft by a vacuum pump where the treated water first passes through the membrane and into the disk before entering the shaft.A picture of the pilot plant and illustration of the principle of the membrane disk arrange-ment and mounting on the shafts is shown in Figure 1.The membrane disks are made with a polysulfone ultrafiltration (UF)membrane that has a molecular weight cut-off of 750,000Daltons.The pilot plant used in this study has a membrane chamber with a volume of 10L (8.44L effective volume),six membrane disks with a diameter of 210mm mounted on two shafts giving a total surface area of
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0.3m 2.The rotational speed of the membranes can be varied between 0–500rpm.The membrane disks were fully submerged in the membrane chamber.
One of the attributes of the RDM unit is that the performance of the system can be enhanced by rotating the disks at various speeds.Rotating the disks with a low rpm reduces the overall resistance to mass transfer thereby improving the performance of the unit.Rotating the disks with a high rpm generates turbulence that agitates the membrane surface creating a self-cleaning effect that helps reduce fouling and clogging of the membrane.This shear induced cleaning will occur above a rotation speed that is specific to the unit as a function of disk diameter,chamber volume etc.An option of using sponges in the chamber for enhanced cleaning during operation was also investigated.Three different operating modes were chosen to assess the effect on bio fouling control and management;
†continuous rotation with a low rotation speed,enhanced performance,
†continuous rotation (low rpm)with periodic shear rotation (high rpm)for cleaning,†no rotation combined with periodic rotation with sponges,shear induced /mechan-
ical cleaning.
Experimental configuration
The principle of the process is shown in Figure 2.Raw water is first ozonated before entering the membrane unit.Biodegradation of the ozonation by-products takes place in the biofilm that is developed on the membrane disk surface.The experimental configur-ation used in this study is based on the scheme shown in Figure 2but where the ozona-tion step was replaced by feeding the RDM reactor with constructed water representative of ozonated water.The RDM unit was operated with a constant flux
and Figure 1Illustration of the RDM pilot unit and principle of membrane disk
arrangement
Figure 2Process principle and basis for experimental configuration T.Leiknes et al.243
very high recovery (.98%).Most of the time a dead-end mode of operation was used and the excess biomass was only removed periodically in conjunction with a cleaning procedure.This is possible due to the very low biomass production.A biofilter was also operated in parallel to the RDM unit as a reference for the biodegradation to compare the efficiency of the biofilm on the membrane disks for the various operating conditions imposed on the membrane unit.The biofilter was designed as a submerged-attached growth biofilter packed with Kaldnes K1biofilm carriers,and with an EBCT of 30min-utes (Ohme,2002;Leikness et al.,2003).
Feed water composition In the earlier studies the raw water used had a NOM concentration giving a colour of 50mg Pt/1,UV 254-absorbance of 25m 21and TOC of 6mg/1.Detailed investigations of the rate of formation and biodegradation rates of the ozonation by-products formed on this type of water has been conducted in other studies (Melin and Ødegaard.,2000).For consistency throughout this study an artificial feed water was chosen representing ozo-nated raw water used in the previous studies.The dominant biodegradable ozonation byproducts formed have been identified as carboxylic acids,ketoacids and aldehydes,and their ratios and respective concentrations for the typical raw water used in this study have been reported.The artificial feed water was therefore constructed by adding a mixture of these compounds to tap water where this addition of TOC represents the biodegradable portion of the total TOC in the constructed water.Two levels of TOC addition were investigated representing raw water with a relatively low and high TOC content respect-ively.The concentrations and ratios of the compounds added and the composition of the feed water are given in Table 1.The background TOC concentration in the tap water was measure as 2.63^0.08mg/1and was found not to be biodegradable under the conditions tested.The biodegradable TOC in the two feed waters tested where therefore 10%and 40%of the total TOC respectively.Experimental analysis The performance of the membrane module was determined by measuring the transmem-brane pressure (TMP)for constant flux operation.The development of TMP for different fluxes was measured continuously using an online pressure transducer connected to a data acquisition system the control panel.Permeate water flow rates were measured with Table 1Comparison of biodegradable constituents added to make artificial feed water Composition of biodegradable compounds used Low level concentration of TOC (m g/l)High level concentration of TOC (m g/l)%of mixture added Carboxylic acids Formic 78.2469.326%Acetic 98.3589.733%Oxalic(dihydrate)47.6285.516%Ketoacids Glioxylic 29.2175.010%Pyruvic 28.6171.79%Aldehydes Formaldehyde 12.072.04%Glyoxal 8.349.63%TOC sources Concentration of TOC (mg/l)Total TOC (mg/l)%of total TOC Background TOC (tap water) 2.63^0.0822Organic mixture added (low)0.302 2.93^0.0810%Organic mixture added (high) 1.813 4.44^0.0840%
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aflowmeter,also connected to the control panel,and the water temperature was logged continuously with a temperature transducer.The water treatment efficiency,i.e.NOM reduction,was measured by analyzing the removal of TOC in the RDM unit.
Results and discussion
The RDM unit wasfirst subjected to a conditioning period to ensure that a viable and effective biofilm was established on the membrane disk surfaces.After approximately 500hours of operation a stable condition was established and the variations in operating modes were initiated.Thefirst mode of operation consisted of adding a low concentration of the organic mixture and maintaining a constant low rotation of10rpm on the mem-brane disks.Results from the experiments are shown in Figure3.During thefirst phase a low level concentration of organic mixture was added to the system,giving a biodegrad-able content of10%of the total TOC.Under these conditions an average decrease in the total TOC of8–9%is measured,and the decrease represents biodegradation of the organic compounds added to the constructed feed water.A very low and insignificant increase in the TMP is also observed for the period of operation which most probably is due to the low concentration of biodegradable organic compounds in the reactor and more an artifact of the low production of biomass rather than the operating condition. The results indicate that this mode of operation is suitable for waters with a low concen-tration of biodegradable TOC and that the fouling rate correlates to the growth of the bio-film.The effect of biofilm growth rates on the membrane fouling rate was investigate by increasing the biodegradable TOC content as defined by the high level concentration of TOC in Table1.From Figure3it is apparent that the increase in biodegradation also gives an increase in the TMP development due to increased growth of the biofilm.Under these conditions the organic mixture added is about40%of the total TOC in the feed water and an average of25–30%of the total TOC is removed.After215hours of oper-ation the maximum operating range for TMP was reached(0.5kg/cm2).The low rotation of the membrane disks was found not to be sufficient to control biofouling and other options of fouling control are necessary.The second mode of operation was therefore introduced where periodic cleaning of the membrane was done by shear rotation(high rpm).
In the second mode of operation a daily cleaning procedure was implemented which involved rotation of the membrane disks at800rpm for15minutes.After a cleaning cycle the excess biomass in the membrane chamber was drained out.The effect of this cleaning cycle on the TMP development is shown in Figure4.The efficiency of the method was not as good as expected compared to previous results using this technique (Kimura,2000;Ohme,2002).The fast rotation caused shear forces across the biofilm sur-
face resulting in cleaning of the membrane surface and reducing fouling,however,
the
Figure3TMP development with time as a function of TOC removed T. Leiknes et al.
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decrease in the TMP after a shear induces cleaning was only between 0.02–0.12kg/cm 2.TMP remained subsequently relatively high but the cleaning procedure helped to main-tain TMP between 0.40–0.55kg/cm 2for a certain period of time.The average reduction around 25%of total TOC was consistent for the period of operation.After 291hours of operation with these conditions it is apparent that the procedure was not sufficient in con-trolling and maintaining the membrane fouling.The third mode of operation was there-fore introduced.Initial tests using sponges inside the membrane chamber while rotating the disks to induce mechanical cleaning of the biofilm showed that a very efficient cleaning of the membrane surface could be achieved (Leikens et al.,2003).A third mode of operation was therefore applied where the system was operated in a dead-end mode with no rotation and periodically cleaning the membranes by carefully rotating the disks with sponges in the chamber.Excess biomass was drained and removed from the membrane chamber after a cleaning cycle.The cleaning procedure was only applied when the TMP reached the set maximum value of 0.5kg/cm 2.A rotation speed of 425rpm lasting for 2minutes was found to be sufficient reduce the TMP from its maximum vale of 0.5kg/cm 2to a starting value of 0.15kg/cm 2.The starting value was chosen as this TMP appears to represent a level of biomass that is necessary to maintain acceptable biodegradation of the organic mixture added to the feed water (reference Figure 1).A volume of ,2L of sponges (dry,loosely Packed)was added to the 10L membrane chamber.The sponges were made from a soft polyurethane textile sheet with the aim of preventing mechanical wear on the membrane surface,and cut into small cubes of about ,30mm 3.No surface damage or abrasion on the membrane disks was observed for the duration of this study and the effect of long term effects using this cleaning method has not yet been assessed.Results shown in Figure 5indicate that the TMP development between cleaning cycles is a function of the biofilm growth due to biodegradation of the organic compounds added to the feed water.The slope of the line of best fit through the TMP development between each cleaning cycle represents the fouling rate for the conditions tested,where the fouling rates for the three cycles shown are 2.1£1023, 2.2£1023and 1.6£1023kg/m 2.h respectively.The duration of each cycle is also similar for the experimental runs as shown.The reduction of total TOC in the reactor remained around the same value of 25%throughout the experiments indicating that a sufficient amount of biofilm was kept in the reactor at any given moment to remove the biodegradable portion of the TOC.With an efficient cleaning cycle using sponges a sustainable production of clean water can be maintained.One of the objectives of this study was maintenance of the biofilm on the membrane to obtain the necessary biodegradation of the biodegradable NOM.On average,a
TOC
Figure 4TMP development after introducing periodic shear induces cleaning
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reduction of around 25%was obtained during the study.To evaluate the efficiency of the biofilm in the membrane unit a conventional biofilm filter was operated in parallel to the RDM unit;diameter 14cm,media height 200cm,EBCT 385minutes.The TOC removal in this filter was also on average around 25–30%confirming that maintenance of the bio-film in the RDM unit was successful given the variations in operating modes tested.Conclusions
The results from this study demonstrate that ozonation/biomembrane filtration is an inter-esting method for treatment of surface water with high NOM concentrations.Ozonation is an efficient method of removing colour in water sources with high concentrations of NOM,however,biodegradable ozonation byproducts will also be formed increasing the biogrowth potential in the distribution network.The easily biodegradable byproducts are readily removed by biologically active filters but a particle separation step is required to remove the excess biomass formed in the biofilter.A rotating membrane disk bioreactor has been investigated in this study as a process option for designing a drinking water treatment plant based on an ozonation/biofiltration process.Operating criteria requires maintenance of the biofilm on the membrane disks to obtain the necessary biodegradation of the biodegradable NOM while controlling biofouling and managing an acceptable water production (i.e.flux).
A constructed feed water was used in this study where known concentrations of a mix-ture of biodegradable ozonation byproducts were added to tap water.Two types of feed were tested,one containing a low level concentration of the mixture (10%of total TOC,
2.9mg/1),and alternatively with a high level (40%of total TOC,404mg/1).TOC measurements showed on average 10%reduction of TOC in the first case and 25–30%in the second indicating that essentially all of the added compounds in the feed water were removed by the biofilm.The TOC removals were consistent throughout the experimental period.Both the amount of biomass and activity was upheld for all modes of operation tested to achieve the necessary biodegradation.
The RDM pilot plant was operated with three different modes of operation;enhanced performance by continuous rotation of the disks with low rpm,addition of periodic clean-ing by shear induced rotation,and finally cyclic operation using sponges for mechanical cleaning combined with disk rotation.A design flux of 201/m 2h with a TMP operating range of 0.15–0.50kg/m 2was chosen for the study based on preliminary tests of the pro-cess.With enhanced performance biofouling by biofilm growth on the membrane disks was maintained,however,the fouling rate increased when the concentration of biodegrad-able organics increased.Introducing a periodic shear induced cleaning by increasing
the Figure 5TMP development and cleaning cycles combining rotation with sponges
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rotation speed of the disks was not sufficient to clean the membranes and control biofoul-ing.Fouling control and cleaning of the membranes using small sponges and rotation of the membrane disks was shown to be very efficient.Operating cycles of 150–200hours were maintained indicating that a weekly cleaning interval lasting 10–15minutes can be achieved under the conditions tested.The overall results indicate that the RDM can favor-ably be used in an ozonation/biofiltration process by carrying out both biodegradation as well as biomass separation in the same reactor.
Future work will include investigation of biofouling as a function of organic loading rates and biofilm growth rates,alternative operating modes and a better understanding of the effect of the cleaning cycles on the system performance,improving the membrane module design,designing a more efficient membrane reactor,and optimizing modes of operation to minimize membrane fouling.
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