Microfiltration for Treatment of Waste Filter Washwater at a North

Microfiltration for Treatmentof Waste Filter Washwater at a

North Jersey Surface Water Treatment Plant

Michael FurreySupervisory Chemist, The North Jersey District Water Supply Commission, Wanaque, New Jersey

James Schaefer, P.E.Vice President, Water Processing, Pall Corporation, East Hills, NY

Matthew GehoWTP Analyst, The North Jersey District Water Supply Commission, Wanaque, New Jersey

Thomas GalloR&D Engineer, Pall Corporation, Port Washington, New York

1.0 BACKGROUND

The North Jersey District Water Supply Commission (Commission) serves about 2 millionconsumers from its 210 mgd water treatment plant (WTP) in Wanaque, New Jersey, about 25miles northwest of Manhattan. Raw water is obtained primarily from the adjacent WanaqueReservoir, and the nearby Pompton and Ramapo Rivers. The Wanaque WTP providesconventional treatment with chlorine as primary disinfection. Waste filter washwater andsupernatant from cleaning the settling basins is recycled to the head of the WTP after passingthrough a holding/equalization basin. Settling basin sludge flows to either the ResidualsTreatment Facility or a sludge lagoon. The lagoon also accepts decant water from sludgedewatering. Lagoon supernatant is recycled to the reservoir. Occasionally, the recycle flowsdisrupt the operation the WTP and impact finished water quality. A site plan for the WTP ispresented on Figure 1.

Because of these impacts on finished water quality and the anticipated new regulations forhandling waste filter washwater, the Commission has investigated alternatives to better handlethese flows and improve finished water quality. Based on successful pilot testing at anotherfacility, one alternative is to treat the waste filter washwater and decanted settling basinsupernatant with microfiltration, and to use the microfiltered water as potable water or to recyclethis water to the head of the WTP. The concentrated solids from the microfiltration unit wouldbe discharged to the Residuals Treatment Facility or the lagoon. This paper presents the resultsof pilot testing microfiltration for this application and a preliminary engineering evaluation ofalternative plans.

1.1 New Regulations

USEPA�s Long Term 1 Enhanced Surface Water Treatment and Filter Backwash Rule proposedin April 2000 requires utilities to conduct a self-assessment concerning the impacts of recyclingwaste filter washwater on the main treatment process. The proposed regulations for recycling

(C) 2000, American Water Works Association, Water Quality Technology Conference Proceedings

waste filter washwater and sludge supernatants seek to maintain high finished water quality byrecycling these flows to the head of a conventional WTP prior to coagulant addition or byproviding other suitable treatment, such as equalization or settling prior to recycling. Theserequirements are less stringent than the requirements anticipated a year ago.

Recent changes to the Surface Water Treatment Rule requiring 2 log removal ofCryptosporidium, or higher depending upon the occurrence of Cryptosporidium, set a morestringent standard for any system treating waste filter washwater for direct use as potable water.The requirement for 2 log removal of Cryptosporidium and lower filtered water turbidity maymake continued recycling with the existing holding basin impractical because of its impacts onthe treatment process.

Figure 1Wanaque WTP Site Plan


1.2 Existing Facilities

The Wanaque WTP provides conventional treatment with coagulation, flocculation, settling, anddual media filtration and uses chlorine for oxidation at the rapid mixer and for primarydisinfection. The WTP has six flocculation-sedimentation basins and 13 dual media filters (Fourflocculation/sedimentation basins placed in service in 1981 as enclosed, two level basins withoutsludge collection equipment). By the end of 2000, these four basins will be equipped withsludge collectors. The two other basins were constructed in 1993 as open, upflow basins withtube settlers and sludge collectors. Alum is fed at the rapid mixer as the primary coagulant witha cationic polymer added at the flocculation basins as a coagulant aid. The WTP also usespotassium permanganate as a preoxidant and lime for final pH adjustment.

Waste filter washwater is discharged to the Waste Washwater Holding Basin on the north side ofthe WTP, which equalizes these flows. The average flow of waste washwater is approximately3.5 mgd, while the peak day flow recently has been 6 mgd. The equalized waste washwater ispumped to either the main process flow before the rapid mix chamber, the equalization tank atthe Residuals Treatment Facility, or the sludge lagoon. During manual cleaning of the settlingbasins, basin supernatant is discharged via a separate drain to the same holding basin andrecycled to the head of the WTP. Sludge from cleaning the settling basins is discharged to thesludge lagoon for removal and storage of the residuals. The clarified supernatant from thelagoon is recycled to the reservoir. Extensive monitoring of the entire process is required inorder to maintain high finished water quality.

Because of the relatively high waste flows and the large amount of solids already in the lagoon,the lagoon supernatant contains high suspended solids and turbidity, which adversely impacts theperformance of the WTP when it is recycled. With the recent construction of the ResidualsTreatment Facility, the Commission is working towards removing the sludge lagoon from theoverall operation of the WTP. Recycling larger volumes from the decanting process to the headof the plant is a significant part of the effort.

2.0 MICROFILTRATION PILOT TESTING

During May 1999, the Commission initiated a pilot test of microfiltration to demonstrate thefeasibility of processing the waste filter washwater and settling basin decant to produce potablewater, or at least water suitable for recycle in the WTP. Pilot testing was performed by theCommission and Pall Corporation with one of Pall�s standard pilot rigs, which are designed forautomatic operation and continuous data collection of important parameters, such as turbidity,pressures, and flows. The Commission provided support to install the pilot unit, on-line particlecounters to analyze feed and filtered water, and laboratory analysis of basic water qualityparameters. Unusual analytical work, such as analysis of Giardia and Cryptosporidium, wasperformed by an outside laboratory.

2.1 Study Program Goals and Objectives

The pilot program was designed to demonstrate reliable performance of the microfiltrationsystem and excellent filtered water quality. Based on preliminary discussions with the New


Jersey Department of Environmental Protection (NJDEP), filtered water from microfiltration ofwaste filter washwater and settling basin decant may be used as potable water, if: 1) the watermeets the appropriate water quality standards; and 2) the treatment system can provide 5 logremoval of Giardia and 4 log removal of Cryptosporidium to assure adequate disinfection. Thedesign of the treatment facilities should provide the level of protection required by the SurfaceWater Treatment Rule and should maximize the use of existing facilities. Because iron andmanganese in settled solids can dissolve under anaerobic or low pH conditions, themicrofiltration system must also be capable of removing these metals.

The water quality goals for the microfiltered water, as presented in Table 1, meet or exceedNJDEP standards for drinking water.

Table 1Filtered Water Quality Goals

Parameter GoalTurbidity <0.1 NTUColor < 5 cuAluminum <0.05 mg/lIron <0.05 mg/lManganese <0.05 mg/lParticle Counts 4 log removal for >2µm

2.2 Pilot Facilities and Operations

In order to have a continuous supply of wastewater and a place to discharge the microfilteredwater and concentrate stream, the pilot unit was installed in the Waste Washwater Pump Station,adjacent to the Waste Washwater Holding Basin (see Figure 2). Because the water level in theholding basin varied significantly, a submersible pump was lowered to about 1 foot from thebottom of the basin to supply mixed wastewater to the microfiltration pilot unit. Afterprocessing, the filtered water and concentrate streams were discharged back into the nearestcorner of the holding basin.

The microfiltration pilot unit was a standard rig, which simulates full-scale operations, with a 3�module. The waste washwater from the holding tank was collected in a small tank on the unitand pumped through the 0.1 µm polyvinyldenefluoride (PVDF) membrane at a flux of 20 gallonsper square foot of membrane surface per day (gfd). The membrane was reverse filtered(backwashed) every 20 minutes for 30 seconds, and air-scrubbed every 40 minutes for 2 minutesto remove solids and maintain acceptable headlosses, or transmembrane pressures (TMPs).Under these operating conditions, system recovery was 94%. The reverse filtration flow was fed10 mg/L of chlorine to control biological growth on the membrane surface and to help removesome of the other foulants. A small amount of the foulants was not removed by reversefiltration, so the TMP gradually increased with time until it reached 28 to 30 psi. At that time,the membrane was chemically cleaned to remove the �attached� foulants and to restore the initialclean TMP. Chemical cleaning for a microfiltration system normally is performed once every 4to 6 weeks. Chemical cleaning typically is a 2 to 3 hour process in which citric acid and caustic(sodium hydroxide)/chlorine solutions are sequentially fed into and recirculated through the


membrane. This cleaning is accomplished without removing the module from the system and isreferred to as a �CIP� (clean in place).

2.3 Pilot Study Period and Monitoring Program

The pilot study began May 23, 1999 and ended July 27, 1999. During this period, the settlingbasins were cleaned twice and the membranes were chemically cleaned once.

Pertinent flows, pressures (inlet, outlet, etc), water temperature, and influent and effluentturbidities were continuously monitored and recorded by the pilot unit�s instrumentation andcontrol system. A manual monitoring program was implemented to collect water quality andoperating data to evaluate the performance of the microfiltration system, as shown in Table 2.

Particle count data were collected using the Commission�s two on-line Hach WPC1900 particlecounters. Both counters were set at one minute sampling intervals and at a continuous flow of200 ml/min. Data was collected and stored in a dedicated PC.

Figure 2Pilot Unit and Installation


Table 2MF Pilot Study Monitoring Program

Parameters Location FrequencyFlow Filtered, Recirculation, &

Reverse FiltrationContinuous

Pressure Inlet, Recirculation, & Outlet ContinuousTurbidity Raw & Filtered Daily & ContinuousTemperature Raw ContinuouspH, Conductivity, Color, & Total& Dissolved Metals (Aluminum,Iron, & Manganese)

Raw & Filtered Daily

Alkalinity, Hardness, Chlorine,TOC, & Total Coliform

Raw & Filtered Occasional

Particle Counts Raw & Filtered OccasionalGiardia & Cryptosporidium Raw & Filtered TwiceTHM & HAA Formation Filtered Once

3.0 PILOT STUDY RESULTS

3.1 Feed Water Characterization

General - Waste washwater and settling basin supernatant collected in the holding basin varied insuspended solids concentration depending on the treatment plant�s operating mode. If thecontents of the holding basin were undisturbed for a period of time, most of the solids settledbelow the intake of the sump pump feeding the microfiltration pilot unit and in turn, reduced thesuspended solids in the feed to the membrane. Conversely, during and shortly after a dischargeinto the holding basin, which created high turbulence, the suspended solids in the feed to thepilot plant increased significantly. This effect was evident in the feed turbidity and TMP datacollected by the pilot rig�s on-line sensors (see Figure 3). The frequency of the WTP�s filterbackwashes depended on the plant�s raw water quality and operating procedures. In addition toregular filter backwashing, periodic cleaning of the settling basins was performed on roughly amonthly schedule, typically spanning a two-week period. During the pilot test period, the basinswere cleaned on June 1-17, and July 14-28. These basin cleanings discharged unusually highlevels of suspended solids into the holding basin and affected the quality of the feed water to thepilot plant to an even greater degree than the filter backwashes.

Feed water quality after a period of quiescent settling was sampled on 5/29/99, and is comparedto the average of the weekly samples collected during the testing period in Table 3.

Turbidity � Based on the continuous turbidimeter data, the average feed turbidity during thetesting period was 30 NTU. According to Figure 3-, the feed turbidity ranged from 2 to 5 NTU,after a period of settling. The on-site feed water turbidity data was similar to the data from theon-line turbidimeters on the pilot rig, as shown in Table 4. The water quality summarized inTables 4 and 5 represent periods of uninterrupted pilot plant operation or significant changes infeed water quality.


Table 3MF Pilot Plant Feed Water Characterization

Parameter(mg/L unless notedotherwise)

GrabSample5/29/99

Average5/24/99 �7/02/99

Parameter(mg/L unless notedotherwise)

GrabSample5/29/99

Average5/24/99 �7/02/99

Turbidity (NTU) ND 30 Iron - Total 0.18 0.39Color (units) ND 2870 - Dissolved 0.05 0.02TOC 4.0 6.5 Manganese � Total 0.55 2.0St. Plate Count (/ml) 23,000 ND - Dissolved 0.05 0.37pH (lab) 7.6 7.3 Aluminum � Total 2.9 11.1Suspended Solids 9 ND - Dissolved ND 0.3Total Solids 190 ND Calcium 16 NDUV-254 ND 0.0369 Magnesium 3.6 ND

* ND = No data available

Table 4Feed and Filtered Water Turbidity, Color and pH Data

Turbidity (NTU) Color (units) pH (units)Period Avg. Max. Avg. Max. Avg. Min. Max.

Feed Water5/24 - 5/31 3.5 6 36 88 7.2 6.9 8.3

6/1 - 6/3 67 83 >1000 >1000 6.9 6.8 7.06/9 - 6/24 9 31 97 483 7.1 6.4 8.3

6/28 - 7/07 12 40 >1000 >1000 7.4 6.7 9.37/14 - 7/29 55 153 >1000 >1000 7.8 6.5 10.4

Filtered Water5/24 - 5/31 0.28 0.47 3.3 7 7.8 6.9 9.1

6/1 - 6/3 0.18 0.22 5.5 7 6.9 6.8 7.06/9 - 6/24 0.20 0.26 5.8 16 7.1 6.4 7.1

6/28 - 7/07 0.44 0.4 6.1 18 7.5 7.0 8.57/14 - 7/29 1.2 8.9 11 66 7.6 6.4 9.8

Feed turbidity varied according to the schedule for filter backwashes. During a backwash, thewaste washwater flowing into the holding basin was very turbid and created high turbulence inthe basin, which kept feed high. Actual peak turbidities were much higher than the recordedvalues because the continuous turbidimeter on the pilot rig was set to a maximum value of 20NTU and was occasionally blocked by solids. When the turbidimeter inlet was clogged, lowturbidities were recorded even when the TMP increased sharply, such as on 6/20 � 6/21. On-sitesampling data, which were as high as 153 NTU, were a better indication of feed turbidity at thesetimes. Periods of low feed turbidity allowed the microfilter to operate successfully at the 20-gfdloading rate and recover from the periodic slugs of high turbidity feed.

Color - The average feed water color was also very high at 2870 units. Most of the color isrelated to the high suspended solids loads in the feed water. Removal of the solids reduced colorto much lower levels.


Iron, Manganese and Aluminum - High concentrations of total and dissolved iron, manganeseand aluminum were present in the feed water, as shown in Table 5. Precipitates of these metals,such as hydroxides and oxides, will be removed by microfiltration; however, the dissolvedfraction will pass through the microfiltration membrane. Dissolved iron, on average, was low,while the average dissolved manganese was up to 0.38 mg/l. The average dissolved aluminumwas approximately 1 mg/l. Subsequent oxidation of the dissolved manganese and precipitationof aluminum will create particles and colored water after the microfiltration process. Properoxidation and pH control prior to microfiltration will be required to control these metals.

Table 5Feed Water Metals Data (Total [Dissolved])

Iron (mg/l) Manganese (mg/l) Aluminum (mg/l)Period Avg. Max. Avg. Max. Avg. Max.

5/24 - 5/31 0.10 [0.01] 0.23 [0.01] 0.20 [0.02] 0.30 [0.03] 1.1 [0.39] 1.6 [1.0]

6/1 - 6/3 0.8 [0.00] 1.1 [0.00] 4.6 [0.06] 5.9 [0.08] 28 [0.03] 34 [0.03]

6/9 - 6/24 0.20 [0.02] 0.6 [0.07] 1.5 [0.38] 4.4 [2.7] 6.0 [0.18] 16 [1.0]

6/28 - 7/07 0.21 [0.01] 0.7 [0.04] 1.0 [0.03] 3.6 [0.10] 5.8 [1.5] 18 [0.7]

7/14 - 7/29 0.6 [0.00] 0.9 [0.00] 3.0 [0.01] 5.6 [0.02] 15 [5.1] 26 [1.1]

Data for particle counts, Giardia and Cryptosporidium, and disinfection by-products arepresented at the end of the next section.

3.2 Microfiltration Results

The performance of the microfiltration rig was very consistent during the two month studyperiod. Typical performance data for headloss across the membrane (TMP), feed and filteredwater turbidity, and flow during a nine-day period in June are presented in Figure 3. This periodof operation was selected to include data during a basin cleaning (a period of high feed turbidity,6/16-6/17), and to show the system�s ability to clean the membrane when the feed turbidity waslow. Periods of operation can be identified by the filtered water or permeate flow rate, whichwas typcially 1 gpm.

Turbidity - Filtered turbidity according to the on-line turbidimeter was 0.02 to 0.04 NTU. As thetest progressed, some iron, manganese and aluminum solids were precipitated, as evidenced bythe reddish brown discoloration of clear tubing and some floc accumulation in the filtered watertanks. On-site filtered water turbidity, as presented in Table 4, was significantly higher than theon-line data. This discrepancy between the two sets of data is more fully explained below, in thesection which evaluates the iron, manganese and aluminum data.

TMP rose and fell in a direct and immediate relationship with fluctuations in the feed turbidity.After a slug of high turbidity feed, the TMP gradually decreased to the pre-slug value as reversefiltration and air scrubbing removed the solids from the surface of the membrane. Therobustness of the system was exhibited by its ability to handle periods of high turbidity loadingand still recover to continue successful treatment, while maintaining less than 0.1 NTU filteredwater turbidity.


Microfiltration Performance - June 16-24, 1999 Feed Turbidity, Filtered Water Flow, and TMP

0

5

10

15

20

25

30

6/16 6/17 6/18 6/19 6/20 6/21 6/22 6/23 6/24 6/25

Date/Time

Feed Turbidity (NTU)Filtered Water Flow (Gpm)TMP (psid)

Microfiltration Performance - June 16-24, 1999Feed and Filtered Turbidity

0

5

10

15

20

25

30

6/16 6/17 6/18 6/19 6/20 6/21 6/22 6/23 6/24 6/25

Date/Time

Feed Turbidity (NTU)Filtered Turbidity (NTU)

Figure 3Microfiltration Performance

June 16-24, 1999


Color - Filtered color was generally less than 7 units. High color values were associated withhigh turbidities. If the filtered turbidity was low, the filtered color also was low.

Iron, Manganese and Aluminum - Removal of metals depended upon whether the metals wereprecipitated or dissolved before reaching the microfilter. High filtered concentrations of iron,manganese, and aluminum represented the dissolved fraction, which passed through themembrane (see Table 6). The dissolved metals were present because of anaerobic or low pHconditions in solids settled on the bottom of the settling and holding basins. When these solidswere disturbed, such as during a settling basin cleaning or when waste washwater wasdischarged into the holding basin, some of the dissolved metals were mixed into the feed waterand passed through the membrane. Post precipitation of the metals was caused by aeration in thedownstream piping and small amounts of chlorine that were added to the backwash water as partof the microfiltration system design. The post precipitation was visible over time in clear filteredwater piping and also in the backwash supply tank. On-site samples had precipitates from thehigh dissolved metals concentration, resulting in high on-site filtered turbidities. Precipitatedsolids were able to reach on-line instruments (turbidimeter and particle counter) and samplecollection ports.

Table 6Filtered Water Total Metals Data

Iron (mg/l) Manganese (mg/l) Aluminum (mg/l)Period Avg. Max. Avg. Max. Avg. Max.

5/24 - 5/31 0.04 0.13 0.01 0.08 0.5 1.86/1 - 6/3 0.01 0.02 0.05 0.08 0.0 0.0

6/9 � 6/24 0.03 0.20 0.37 2.7 0.1 0.56/28 - 7/07 0.01 0.02 0.03 0.03 0.4 1.97/14 - 7/29 0.01 0.02 0.01 0.03 2.5 10.3

This situation became worse as the filtered water with precipitates and floc was used tobackwash the microfilter. These solids were pushed onto the downstream (clean water) side ofthe membrane during backwashing, only to be removed after the microfilter returned to filteringoperation. Immediately after a backwash, the filtered water turbidity and particle counts spikedand then decayed with time. The solids in the filtered water increased particle counts and gavethe impression that the membrane had failed.

Oxidation before microfiltration is required to precipitate the dissolved metals and allowed themto be removed by the filter, like in the main WTP.

Particle Counts - Particle counting was also used to measure microfiltration efficiency bycollecting data on the waste washwater flow before the holding basin and on the filtered waterdischarge. Particle concentrations in the feed water exceeded the instrument�s upper detectionlimit and caused that sensor to mis-read the true values. Subsequently, the feed water wasdiluted to obtain accurate results. The feed water particle counts were on the order of 106 to107/ml for particle sizes greater than 2 microns (µm). Filtered water particle counts ranged from1 to 104 particles/ml greater than 2 µm. Particle removals were estimated to be from 1.8 to 5.8logs.


After reviewing the other operating data, the high filtered water particle counts appeared to berelated to the spikes in filtered water turbidity, iron, manganese, and aluminum. These resultsindicated the most likely cause was post precipitation of dissolved metals creating floc andparticles. Additional pilot testing with pre-oxidation is required to confirm that themicrofiltration system adequately removes properly oxidized metals.

Giardia and Cryptosporidium - No Giardia or Cryptosporidium were detected in two sets of feedand filtered water samples, therefore, a removal rating was not calculated. The Pallmicrofiltration system has achieved 4 to 6 log removals (depending upon the feed concentration)in other third party challenge tests; similar removals were expected for this test.

Disinfection By-Products - One set of feed and filtered water samples was analyzed fortrihalomethane and haloacetic acid formation potential (THMFP and HAAFP). The testconditions were 7 days of reaction time at pH 8 and 20o C with a chlorine dose of 10 mg/L. Thefeed and filtered water chlorine demands were 9.3 mg/L and 4.8 mg/L. Microfiltration achievedmoderate removals of 17% and 19%, as shown in Table 7. The DBP results are similar to theavailable TOC data, which indicated removal of TOC associated with particles to a reasonablyconsistent level of dissolved organic carbon (DOC). Specifically, TOC removals were 15 to25% when the DBP samples were collected.

Table 7DBP and TOC Data

THMFP (µµµµg/L) HAAFP (µµµµg/L) TOC (mg/L)Date Feed Filtered Feed Filtered Feed Filtered

5/26/99 - - - - 3.4 3.26/2/99 - - - - 14.8 2.46/8/99 - - - - 10.8 2.8

6/16/99 - - - - 3.9 3.36/23/99 - - - - 2.1 1.87/14/99 151 123 226 187 2.4 1.87/21/99 - - - - 2.9 2.47/28/99 - - - - 3.6 2.1

CIP Frequency - During this pilot program, one chemical cleaning was performed during July,and the TMP was restored to its original clean value. The CIP frequency was in the 4 to 6 weekrange. With better quality feed water, the cleaning frequency may increase.

4.0 PROPOSED WASTE WASHWATER TREATMENT SYSTEM

4.1 Design Considerations

The design of a full-scale microfiltration system for treating waste filter washwater and settlingbasin decant generated by WTP operations must meet the following objectives:

• Handle the spikes of turbidity and suspended solids


• Tolerate oxidant residuals for control of iron and manganese• Provide excellent removals of pathogens, such as Giardia and Cryptosporidium• Produce filtered water with low turbidity.

A microfiltration system removes contaminants that are in the form of particles. For example,iron and manganese must be oxidized and precipitated, in order to be successfully removed.Giardia and Cryptosporidium are virtually completely removed because their particle size isgreater than the 0.1 µm pore size of the microfiltration membrane.

Spikes of Turbidity and Suspended Solids - The existing holding basin that receives the flows ofwaste filter washwater and settling basin decant can be redesigned to better distribute flow andremove some of the suspended solids. Currently, heavier solids settle and accumulate in thebottom of the holding basin and must be periodically removed. The recommended modificationsto the holding basin include new inlet piping to evenly distribute flow across the entire basin andsludge removal equipment to frequently remove accumulated solids and discharge the solids tothe existing lagoon. The easiest-to-install sludge removal equipment for this basin will be thevacuum type on rails or tracks along the floor of the basin similar to the sludge collectors beinginstalled in the 1981 settling basins.

Iron and Manganese Control - Solids which contain iron and manganese precipitates accumulatein the bottom of the settling basins or the holding basin. The iron and manganese can dissolveunder the anaerobic or low pH conditions that develop in the solids accumulation, especiallywhen water temperatures are high. The dissolved iron and manganese will be recirculated to thehead of the treatment plant and result in operational problems and degraded filtered waterquality. The installation of sludge removal equipment in the holding basin will reduce theamount of dissolved iron and manganese, however, some dissolved iron and manganese willcontinue to be generated in the settling basins. An oxidant feed to the waste washwater andsettling basin supernatant flows is recommended. Because manganese is likely to be present, theoptimum oxidant will be potassium permanganate. The potassium permanganate should be fedinto the inlet piping into the holding basin so that the oxidized iron and manganese canprecipitate and settle in the holding basin. The required dosage will depend upon the quality ofthe flows entering the holding basins. For planning purposes, the feed system was sized todeliver about 5 mg/L of potassium permanganate. The actual dose will vary depending on theoxidant demand of the inflows, such as the amount of dissolved iron and manganese, and theeffectiveness of frequent solids removal from the holding basin. Controlling dissolved iron andmanganese will reduce filtered turbidity by eliminating post precipitation.

Membrane Flux - The pilot system operated successfully at a flux of 20 gfd with the regularspikes of turbidity and suspended solids. TMP increased significantly when the spikes occurred,and steadily declined to the pre-spike value with regular reverse filtration and air scrubbing toremove accumulated solids. The chemical cleaning frequency was approximately 3 or 4 months.The long chemical cleaning interval indicated that a higher flux could be sustained if the solidsloading could be controlled more effectively. Longer CIP intervals may be possible by adjustingreverse filtration and air scrubbing parameters to fit the feed water quality. Another microfilterpilot test successfully treated waste filter washwater at a flux of 30 gfd.


The recommended design flux for a microfiltration system depends upon the quality of the feedwater to the microfiltration unit. For feed water, which still will have the spikes of turbidity andsuspended solids, a flux of 20 gfd is recommended. For feed water, which will be betterequalized and settled, a flux of 25 gfd is recommended. In either case, the chemical cleaninginterval is expected to be approximately two months.

The proposed waste washwater treatment system is designed to handle the 3.5 mgd average flowand the peak day flows of approximately 6 mgd, plus the occasional discharges of settling basinsupernatant. Two treatment alternatives were considered:

• Alternative 1 � Microfiltration without Equalization � continue to use the holdingbasin to receive waste washwater and basin supernatant flows and microfilter thecombined flow.

• Alternative 2 � Microfiltration with Equalization � modify the holding basin toremove some of the solids and equalize the waste washwater and supernatant flows, andmicrofilter the combined flow at a higher loading rate.

In either case, the microfiltered water will be used as potable water and microfiltration wasteflows will be piped to the existing sludge lagoon. Each alternative is described below.

4.2 Alternative 1 � Microfiltration without Equalization

Alternative 1 includes the installation of a new microfiltration system to treat the discharge fromthe holding basin and produce potable water. The existing holding basin will continue to receiveand hold the waste filter washwater and settling basin decant flows prior to microfiltration. Nomodifications to the inlet piping or holding basin are required for this alternative. The basis ofdesign for Alternative 1 is summarized in Table 8 and shown schematically on Figure 4.

Table 8Basis of Design for Treatment Alternatives

Flow (mgd) MF Design Criteria Discharges to LagoonAlter-native

Flow toHolding Basin Avg. Peak

Flow(mgd)

Feed Turb.(NTU)

Flux(gfd) Type

Flow(mgd)

Existing Waste Wash-water 3.5 6 - - -

None fromHoldingBasin

-

1 Waste Wash-water 3.5 6 6 30 20 MF Waste-

water 0.2-0.4

2 Waste Wash-water 3.5 6 6 15 25

MF Waste-waterSludge fromHolding Tank

0.2-0.4

0.1-0.2

The design flow for the microfiltration system will be 6 mgd to accommodate the peak wastewashwater flow. The design flow is based on recent improvements in filter operations and


backwashing procedures, and the installation of sludge collection equipment in all six settlingbasins, which will virtually eliminate the flows of settling basin supernatant. The microfiltrationsystem will recover 94% of the water fed into the system; the remaining 6% of the flow,typically 0.2 to 0.4 mgd, will carry the concentrated solids into the existing sludge lagoon.

The holding basin will continue to operate as it has, with some solids settling and accumulatingon the bottom of the basin. Some iron and manganese will dissolve from solids accumulated onthe bottom of the holding basin, so preoxidation with potassium permanganate will be needed tomaintain high filtered water quality.

The required modifications for Alternative 1 include:

• 6 mgd microfiltration system for waste washwater and occasional flows of settling basinsupernatant

• Potassium permanganate feed facilities for preoxidation• New higher head variable speed pumps in the Waste Washwater Pump Station• Piping connections for microfiltered water to the finished water clearwell and

microfiltration wastes to the existing sludge lagoon

Figure 4Process Schematics for

Microfiltration Alternatives


The microfiltration system will consist of six racks with spaces for 100 microfiltration modulesin each rack (see Figure 5). Ninety-four modules will be installed in each rack to provide 6 mgdof capacity at a loading of 20 gfd. The microfiltration system also will include the chemicalcleaning equipment (tanks, pumps, chemical feed and storage), and instrumentation and controlsadequate to control all parts of the system.

The potassium permanganate feed system will use dry potassium permanganate and will consistof a mixing and feed tank, and metering pumps.

The microfiltration system and potassium permanganate feed systems will be housed in a onestory building of approximately 4,900 ft2 using finish materials to be compatible with the otherbuildings at the WTP. The new building will located on the southeast side of the existing WasteWashwater Holding Basin.

The microfiltration system will increase the head requirements for the recycle pumps by about 30psi, so new pumps are required. The new pumps will be variable speed units for energyefficiency and to be compatible with the microfiltration system

Figure 5Plan of Proposed Microfiltration Facility


The filtered water will flow via 24-inch diameter piping to the north end of the existing filteredwater conduit between the Administration Building and the Settling Basins. A 6-inch pipe willconvey microfiltration wastes to the piping to the sludge lagoon at the southwest corner of theWTP.

4.3 Alternative 2 � Microfiltration with Equalization

Alternative 2 is very similar to Alternative 1, except the holding basin will be modified to betterequalize the inflows of waste washwater and settling basin supernatant and remove some of thesuspended solids. Because of the improved feed water quality, the microfiltration system will bedesigned with a higher loading rate, which reduces the size and cost of the equipment. The basisof design for Alternative 2 is also summarized in Table 8.

The modifications to the holding basin will include new inlet piping to be installed as shown inFigure 6. The new 42-inch inlet piping will carry both the waste washwater and the basinsupernatant and distribute the water evenly across the east side of the basin through four 16-inchdiffuser ports. The new inlet pipe will be installed about 5 feet from the east wall and the four16-inch ports will direct water toward the east wall, which will act to dissipate the velocity andspread the flow across the basin. With the better hydraulics, more solids will settle in theholding basin, so a vacuum style sludge removal mechanism will be installed on the bottom ofthe basin to remove the solids as they accumulate. The removed solids will be pumped into thesludge lagoon via an 6-inch pipe.

Figure 6Proposed Holding Basin Modifications


Because of the higher loading rate, the microfiltration system will consist of five racks withspaces for 100 microfiltration modules in each rack (see Figure 5). Eight-nine modules will beinstalled in each rack to provide 6 mgd of capacity at a loading of 25 gfd. The microfiltrationsystem also will include the chemical cleaning equipment (tanks, pumps, chemical feed andstorage), and instrumentation and controls adequate to control all parts of the installation.

The other components for this alternative will be the same as for Alternative 1, except that thebuilding to house the microfiltration system and potassium permanganate feed systems will be aone story building of approximately 4,400 ft2.

4.4 Estimated Costs

The estimated capital costs for the alternatives are summarized in Table 9.

Table 9Estimated Capital Costs for Microfiltration Treatment

ItemAlternative 1 �

without EqualizationAlternative 2 �

with EqualizationMicrofiltration System $3,800,000 $3,100,000Building for Microfiltration System 490,000 440,000New Recycle Pumps 80,000 80,000Pre-Oxidation 50,000 50,000Sludge Collectors - 200,000Piping and Valves 300,000 320,000Electrical and Instrumentation 450,000 400,000Contingencies (10%) 530,000 460,000Total Estimated Cost $5,700,000 $5,050,000

The estimated capital cost for Alternative 1 � Microfiltration without Equalization is $5,700,000for all of the proposed improvements including 10% for contingencies. The microfiltrationequipment consists of the microfilter modules and racks, piping and valves, controls, chemicalclean equipment, and related chemical storage and feed equipment.

The estimated capital cost for Alternative 2 � Microfiltration with Equalization is $5,050,000 or12% less than Alternative 1. The principal differences are the smaller microfiltration system andbuilding to house the system, and the addition of sludge collection equipment.

Estimated operating costs for the microfiltration system are summarized in Table 10. Theestimated costs of power and chemicals are the same for either alternative because the systemwill treat the same average flow. The estimated membrane replacement cost is significantlylower for Alternative 2 because of the smaller number of membranes in the system. Theestimated useful life for the membrane modules is 8 years. The estimated unit operating costsusing an average flow of 3.5 mgd range between $0.26 and $0.31 per 1000 gallons of filteredwater.


Table 10Estimated Annual Operating Costs for Microfiltration System

ItemAlternative 1 �

without EqualizationAlternative 2 �

with EqualizationPower $60,000 $60,000Chemicals 20,000 20,000Membrane Replacement (8 yr life) 320,000 250,000Total Annual Costs for Power,Chemicals, and Membranes $400,000 $330,000Operating Cost for Filtered Water $0.31/1000 gal $0.26/1000 gal

Operating labor will be 2 to 4 hours per day because the system will operate automatically andonly simple chemical feeds are required for microfilter operation

5.0 CONCLUSIONS

The waste washwater and settling basin supernatant were successfully treated withmicrofiltration to produce potable quality water. The microfiltration system with its 0.1 µmmembrane was a barrier to the recycle of Giardia and Cryptosporidium, so adequate disinfectionwas provided by the microfilter. This system will be able to meet anticipated future regulationsconcerning the handling of waste washwater as well as current and future standards for drinkingwater. The estimated capital and operating costs for Alternative 2 � Microfiltration withEqualization are 12 to 15% lower than Alternative 1 - Microfiltration without Equalization. Inaddition, system operation will be more reliable if equalization is provided.

The performance of the microfiltration system with preoxidation with respect to the removal ofparticles and turbidity should be confirmed with additional pilot testing. Pre-oxidation isrequired to precipitate dissolved metals so that the metals can be removed by the microfilter.Longer term pilot testing of the microfiltration system on equalized waste filter washwater andsettling basin decant should be performed to test the feasibility of a 30 gfd loading (or flux) ratethat was successfully pilot tested at another location. The least cost approach, Alternative 2, isbased on a loading of 25 gfd. If the loading were increased to 30 gfd, the estimated capital costswould be reduced by $500,000 to $750,000 because of the smaller microfiltration system andbuilding to house it. Estimated operating costs also would be reduced because membranereplacement costs would be reduced with fewer membrane modules in the system. Theestimated operating costs at 30 gfd would be approximately $0.22/1000 gallons of water filtered.These savings are significant enough to warrant additional pilot testing to optimize the systemdesign.


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Microfiltration for Treatment of Waste Filter Washwater at a North