How to Make Great Sludge Cake When utilized properly, pressure filters bolster operating efficiencies in wastewater treatment, saving time and money. By John Seldon, CCEP, RPP, Temporary Operations and Maintenance

W 'alk, no, run, down to your was t ewa te r t rea tment system and look at the forest of filter presses si t t ing like mute, meta l

beasts as they fail once again to keep up with the "solids" loading you produce in your was t ewa te r t rea tment system.

See if you can find the space for the next one to be added, because you know that has been a reality of operating this sys tem--a lways the call for another press - -and then if you can find the money for pur- chasing it, installing it, and operating it.

Always look for the bright side, right? This must mean that you are doing so much business that the new press is simply a consequence of your own suc- cess; it's par t of the price of doing business. More business, more solids, more presses. Simple math.

You are wrong, wrong, wrong. Pressure filters, from the old plate and frame units

to recessed plate presses, are the most poorly used pieces of environmental equipment in history. And this costs you money, capital, as well as operating funds, plain and simple. Run properly, they are your best friend.

Ber t Sherwood wrote a clear-eyed article "Surviving the Chinese Economic Tsunami" in Metal Finishing's September 2004 issue. He argued for the three C's of "cash, creativity, and courage" as a means of addressing resistance to change. With a respectful acknowledgement to Mr. Sherwood, let's add a fourth "C"--correction.

By correcting historically wrong-headed operating techniques, we optimize wha t we a l ready have, immediately saving hard-earned dollars and estab- lishing new operating baselines from which needed change can be accurately assessed.

The following are a number of myths regarding pressure filters and their operat ion--a profile of ill- tering's poor performance.

Feed mixture: The feed sludge pumped to the press is not characterized with respect to solids con- centrat ion (filterable and dissolved) and flow vol- ume (let alone pumping rate). That is, you don't know the amount of solids you are processing or how much liquid volume you are fi l tering to capture them. You should. Someone, a long time ago, told you

that it didn't mat te r how much liquid you passed through the f l t e r press in capturing the solids. Industrial myth number i is wrong.

It matters. Failure upstream of the clarifier to pro- duce an appropriately concentrated underflow of ill- terable solids costs you long cycle times when filter- ing and long cycle times are costly. They result in more filter presses being installed than are needed.

Conditioning the feed mixture (Part I): Assumpt ions are dangerous. The solids captured by the clar i f ier / thickener tha t you subsequen t ly feed to the press were most likely contacted with a polymer to aid in flocculation, improving sett l ing and solids capture . Assumpt ion and indus t r ia l my th n u m b e r 2. Since these solids have been t rea ted with a polymer a l ready (at clarification), there is no need for addit ional t r ea tmen t prior to filtering. Error. Tilt. Incorrect. Almost universally, the f i l ter ing charac te r i s t i cs for the s ludge feed s t ream are poor and need to be optimized prior to dewatering.

Conditioning the feed mixture (Part II): Industrial myth number 3: Introducing inert filter aid to the feed sludge stream is an effective way to improve filtering characteristics (as a body feed or as a precoat). No. Quick fixes are expensive. Most likely it is a costly suggestion reflecting a failure to optimize the feed stream (see above) with very mod- est polymer conditioning.

The inert filter aid costs you money-- to add to the feed sludge stream and in increased product being shipped for disposal. I am willing to bet you don't know the proportion of disposal costs tha t can be at tr ibuted to the addition of the inert aid and if you figure it out, you will be appalled. Inert filter aid addit ion is an expensive shor tcut replacing the effort for figuring out what really works.

Continuous feed: Another myth- - indus t r i a l - sized number 4. When your wastewater system was set up, someone argued that the sludge stream from the bottom of the clarifier, or thickener, or clarifi- er/thickener (take your pick), could be fed effective- ly and directly to the filter press without the need for addit ional tankage in between. Very wrong. Pressure filter systems are not built for a continu-

36 www.metalfinishing.com

ous feed process, stopping operation only when the press cycle seems to be finished. This is increasing your filtering t ime--and costing you operating effi- ciency and lots of money.

Eight-, 16-, 24-hour press cycle times: This is a beauty, a heartbreaker: Myth number 5. Somehow, in an indus t ry tha t has the sophistication to lay down perfect layers of metal on virtually any sub- strate, the myth has grown up in its wastewater management practices tha t a 24-hour cycle time on its presses is OK. It isn't. It is unconscionable. Feel better if yours are eight hours? Don't. If you are over two hours you are doing poorly. Time is money and you are giving it away.

Set it and forget it operation: Myth number 6 fits right in with number 5. It is the myth that filter presses are simply to be closed, their feed sludge pump turned on, and for eight to 24 hours there is nothing left to do. At tha t point you open the press, scrape out poor qual i ty cake, and s tar t all over again. SOP, right? Wrong. It is a complete waste of a good piece of equipment; capital funds sitting doing next to nothing for most of its working cycle.

Change those fi lter c loths tw ice a year: Myth number 7: Someone inevitably says tha t when old cloths are replaced with new, dewater ing improves. It only seems that way. The gains are transient. And I am willing to bet tha t if you improve your cycle t imes from 24 to 16 hours, it is for only a brief period and then you are back to 24 hours.

It is unlikely tha t it is even the cloth changes tha t are making the difference. If you are chang- ing your cloths more frequently than every five years you are wasting your money. And what a waste. Not only are the cloths expensive but calculate the man- power costs for each exchange, twice a year for five years. Money and manpower wasted.

Fi l ter c lo th rat ing: Myth number 8: Filter cloths are really quite remarkable. The right cloth not only captures your solids but will release them as a solid cake vir tual ly wi thout assistance when dewater ing is at its best (see Figure 1). Large metal hydroxide waffles drop out of a

Figure 1: Sludge cake being released from filter cloth.

press the way waffles pop up out of toasters. The myth is tha t the cloth weave for f i l tering metal hydroxides has to be very t i gh t - - say one to two scfm. Wrong again. If that feed sludge stream is con- ditioned correctly, a much more open weave will work just f ine--say in the 65- to 130-scfm range.

Check with your cloth supplier for a difference in pricing from a tight weave cloth to one much more

J

m

integrat~ d~s~ colI¢ctio~ ard material Mnd!;~g Easy ape~attoe~ maintenance

www.lsindustries.com

Circle 030 on reader information card or go to www.tbru.to/webconnect

December 2004 37

open. In fact, the more open weave will accommo- date any number of different types of sludges, if the feed stream is properly handled.

Feed pressure levels: Myth number 9: Typically, sludge is fed to the press using air operated pumps. (Some newer sys tems are using positive displace- ment pumps, but tha t is a subject for another day.) It is not unusua l for these feed pumps to be oper- at ing at lower feed pressures than the systems are designed for. That is, a system may be designed for a feed p re s su re of 100 to 110 psi and be found operat ing in the 70- to 90-psi range.

First of all, you are again lengthening your cycle t imes and operat ing inefficiently and costing your- self money. Second, you may be sacrif icing the quali ty of cake (dryness) you are get t ing in the end product, resul t ing in more cake overall (it is wet- ter) tha t has to be disposed of at your appropria te landfill , wi th all the assoc ia ted costs. A w e t t e r cake t ranspor ts poorly, as well.

Remember , the feed pump p re s su re is where "pressure fil ter" gets its "pressure"; this is the working pressure for fil tering the solids. The press closing p r e s s u r e - - t y p i c a l l y 3,500 p s i - - s i m p l y seals the plates, prevent ing sludge from squir t ing

out the sides; the feed pump does the f i l ter ing work. Low feed pressure costs you money.

C a k e d r y n e s s : Myth number 10: This is a tough one, but very important . In general it can be said tha t the longer you press, the dryer your cake. This perspect ive tends to draw an operator into the "more is be t te r" f rame of mind; tha t is, 12 hours are bet ter than 6 and 24 hours are bet ter t han 12. It is insidious. Wha t is genera l ly not known is how much t ime it t akes to increase a cake's d ryness [as total solids (% TS)] from one level to ano the r - - say from 18 to 20% TS.

Without addressing the proper operation of a filter press and its sludge feed characteristics, you can still go from 18 to 20% TS- -bu t it will take you a very long time to do so, perhaps the longest portion of the total cycle time. Properly run, a higher total solids cake can be achieved in two hours rather than a 24-hour cycle of filtering a sludge inadequately concentrated or unconditioned.

W H Y ALL THE FUSS? Capital and Operat ional Cost Sav ings Why does all this really matter? By cutting a cycle time from 24 hours to 12 hours, you have doubled

Circle 002 on reader information card or go to www. thru . to /webconnect

38 www.meta l f in ishing.com

the efficiency of the equipment-- in short it can now process twice the volume of sludge. Reduce the cycle time to four hours and you increase the efficiency by a factor of s ix--six t imes more volume can be pressed in one machine in the same period of time. Reducing cycle times stops the need to purchase new or old machines, install them and then operate them poorly as well. I f you have one press at its capacity now, with a 12-hour cycle t ime (CT) and you can reduce its CT to half or better, there is no need for a new unit.

If you have a forest of presses--and many places do--it means that not only will you not need to add another, you can begin to take some out of service. When was the last time you saw tha t happen?

Enough of the incorrect myths of pressure dewa- tering. Let's correct them and bring you the efficien- cy tha t these rather remarkable machines can actu- ally deliver. Consider the following:

Less is better: In general, the more concentrated the feed sludge to the press, the better. Do the m a t h - - a 2% TS (filterable solids) sludge s t ream delivers the same number of solids in half the vol- ume of a 1% TS solids stream. Try to produce a feed solids concentration of up to 4% TS (one quarter the volume of a 1% TS stream) from your clarifier/thick- ener. When the filter press is capturing the solids, they build up on the surface of the cloth first; every ounce of water that . is subsequently treated has to pass though the (ever-increasing) build-up solids prior to passing through the filter cloth and dis- charging as filtrate. The less liquid to push through the better off you are.

Batch versus cont inuous: Isolate a sufficient volume of feed sludge for a particular press volume in a sludge feed tank independent of your clarifier or thickener. Feed your press from here until its press cycle is complete. Pressure filter dewater ing is a batch process best accomplished with a consistent quality feed stream.

Condit ioning: The filter press is meant to cap- ture the filterable solids in the feed stream by fil- ter ing them out wi th a filter cloth covering its recessed plate; however, the solids, after clarifica- tion/thickening, are no longer separating from the water well. When you filter them in this condition you are trying to both separate the solids from the water as well as filter them from the water.

The press is not meant to do this. You can see the effect very quickly as the feed pressure (measured at the feed end of the press, where equipped) increases to its max imum almost as soon as the dewatering cycle begins. And cycle times go out of sight. There, prior to filtering from your sludge feed

Figure 2: Water channeling.

tank, recondition the sludge in the tank with a batch t reatment of polymer. Typically, this can be accom- plished using the same polymer used earlier in clar- ification--the good news being tha t only very low dosages may be required--on the order of a kilo- gram of polymer per metric tonne dry solids or less. This is a small price to pay. There may be a chance to decant some clear water before operating, reduc- ing liquid volumes to be pressed as well.

Operate the press feed pumps: With the feed sludge prepared properly, the rate of pumping to the press can be quite high, filling the press quickly and resulting in filtrate being discharged in large quan- tities very early on. The filtrate may be hazy at first flush, from washing out material left over from emp- tying the previous press cycle, but should clear quickly and stay tha t way

As a rule of thumb, about two thirds of the total volume to be pressed can be filtered in one-third the total press time. Periodically measure the volume pressed either from your feed tank or from the rate of filtrate discharge. Be proactive in this operation.

Ensure the feed pump discharge pressure is operating at the level recommended for the press. Look for cycle t imes- - the time from star t ing the feed pump to shut t ing it off as the cycle is com- plete--from one to no more than three hours.

Manpower: While there is no subst i tute for con- ditioning sludge prior to dewatering, there is also no subs t i tu te for experienced manpower on this equ ipmen t - - e spec ia l ly as cycle t imes shorten. Ins t ead of one operator beginning a cycle and another ending it eight, 12, or 24 hours later, shorter cycle t imes will allow you to schedule spe- cific manpower for establ ished shifts to dewater your weekly sludge in a t imely fashion.

For example, an operator dedicated to the press- es may be able to dewater a week's sludge accu- mulat ion during the day shift, Monday to Friday,

December 2004 39

replacing any number of opera tors deal ing wi th presses at any t ime of the day, through a 24-hour day, seven-day work week. This opera tor will be busy - - se t t i ng up the next batch of sludge, condi- t ioning the sludge, opening and clearing presses, closing and feeding presses a n d - - i m p o r t a n t l y - - monitoring press cycle t imes and volumes run over time. Your facility and the amount of sludge pro- duced will dictate how close you can get to th i s - - with these efficiencies you will go a long way.

Cloth housekeeping: Periodically you may need to wash your filter cloths. Normal use with modest cycle times will provide a cake that forms well and clears the cloths easily when the cycle is finished, leaving little need for washing them at all. However, during a general shutdown (annual, semi-annual), providing them with a power wash (be careful not to damage them) while still on the plates can be a use- ful thing. Poor conditioning or contamination of the sludge with some substance that coats the cloths will require cleaning as needed.

Some firms remove the cloths and send them for commercial cleaning. Removing and then rein- stall ing them is a labor-intensive effort complete with all its costs. Check them regularly for tears and ensure they are and remain installed properly, with- out wrinkles developing across the face of the plate. A good set of cloths should be productive for at least five years.

Plates: The surface of the filter plates provides the channeling for the water passing through the ill- ter cloth to get away, usually to the corners of the press (see Figure 2). These channels may be formed in any number of ways and should be obvious to the eye when you look behind the cloths during a regu- lar inspection.

The channels can become clogged with fines that have managed to get pas t the cloth and become lodged in these water exits.

If you are operating within the one- to three-hour cycle time this should be less of a problem than if you have very long cycle times. Periodically pull back the filter cloths on a few plates and check these channels. If they are plugged, try the high-pressure washer as means of clearing them (removing the ill- ter cloth first, of course).

Some firms have a crew, or hire a crew, to acid wash their units, cleaning both cloths and plates. Operating within more normal cycle times, the need for you to clear plate channels or to hire others to acid wash your system should drop right off.

Pressure gauges: If you don't already have one, install a good-quality pressure gauge on the feed line to the press, close to the sludge feed into the

press. Your equipment manual should indicate the safe, high-end operating pressure for your unit (typ- ically 100 psi), and this gauge will provide you with a good indication of jus t how close you are. If you have conditioned your sludge so that the press can effectively filter it, you may as well use all the pres- sure you are allowed to do the job. The result will be shorter cycle times and drier cake.

Operating profile: For each press you operate, develop a number of operat ing profiles reflecting volume processed against t ime run. This is easily accomplished by charting sludge volume run (or ill- t ra te discharged) against time. Over a number of runs, s tandard profiles will emerge for a range of feed total solids values.

Use these profiles for developing an operat ing manual for each press. In the absence of an experi- enced operator, a trainee or replacement operator will be better able to ensure the presses are run effi- ciently with a well-written manual at hand to point the way.

SUMMARY Pressure filters can produce high-qual i ty sludge cake from a wastewater system's t rea tment process sludge stream in one to three hours. In order to do this, they need to be operated in a batch-process technique, with a feed that has been properly con- centrated, conditioned, and fed at system-designed pressure rates.

Run on a proactive basis by experienced operators; you will minimize the need for adding units. If you a l ready have mult iple units and you adjust your techniques accordingly, you will have presses stand- ing empty, unneeded. And you can forget about pric- ing a new one-- i t jus t won't be required.

Get your cycle times down, way down. Make the best of your current capital equipment and save time and money.

John Seldon, CCEP, RPP (Canadian Certified Environmental Practitioner and Registered Professional Planner) is an independent wastewater contractor/environmental planner whose firm, Temporary Operations and Maintenance, is located at 21 Erieus St., P.O. Box 491, Port Burwell, Ont., Canada NOJ 1T0; 519-874-4213; E-mail, jseldon@hotmail, com. mf

40 www.metalfinishing.com