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Meet- en regelapparatuur bij slibontwatering - Stowa

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<strong>Meet</strong>- <strong>en</strong> <strong>regelapparatuur</strong> <strong>bij</strong> <strong>slibontwatering</strong><br />

<strong>Meet</strong>- <strong>en</strong> <strong>regelapparatuur</strong> <strong>bij</strong><br />

<strong>slibontwatering</strong><br />

Zelf optimaliser<strong>en</strong>de regelunit<br />

Onderzoek op awzi Almere<br />

2002 43<br />

2002 43


Titelpag 2002-43 04-03-2003 17:09 Pagina 1<br />

<strong>Meet</strong>- <strong>en</strong> <strong>regelapparatuur</strong> <strong>bij</strong><br />

<strong>slibontwatering</strong><br />

Zelf optimaliser<strong>en</strong>de regelunit<br />

Onderzoek op awzi Almere<br />

Arthur van Sch<strong>en</strong>delstraat 816<br />

Postbus 8090, 3503 RB Utrecht<br />

Telefoon: 030 - 232 11 99<br />

Fax: 030 - 232 17 66<br />

E-mail: stowa@stowa.nl<br />

http://www.stowa.nl<br />

Publicaties <strong>en</strong> het publicatie-overzicht<br />

2002 43 Stichting Toegepast Onderzoek Waterbeheer<br />

van de STOWA kunt u uitsluit<strong>en</strong>d bestell<strong>en</strong> <strong>bij</strong>:<br />

Hageman Fulfilm<strong>en</strong>t<br />

Postbus 1110<br />

3300 CC Zwijndrecht<br />

Telefoon : 078 - 629 33 32<br />

fax: 078 - 610 42 87<br />

E-mail: info@hageman.nl<br />

o.v.v. ISBN- of <strong>Stowa</strong> rapportnummer<br />

<strong>en</strong> e<strong>en</strong> duidelijk afleveradres.<br />

ISBN 90-5773-201-7


Colofon<br />

Utrecht, 2002<br />

Uitgave:<br />

STOWA, Utrecht<br />

Projectuitvoering:<br />

Eddie Koornneef (DHV Water BV), Cees Visser (Alfa Laval BV), Robbert van der Kuij (DHV<br />

Water BV)<br />

Begeleidingscommissie:<br />

Ad de Man (Zuiveringschap Limburg; vz), Ruud van Dal<strong>en</strong> (Waterschap Veluwe), Hans Moll<strong>en</strong><br />

(Hoogheemraadschap van West-Brabant), B<strong>en</strong> Roelfzema (Waterschap Zuiderzeeland), Hans<br />

Schepman (Waterschap Groot Salland), Ronald van Walrav<strong>en</strong> (Di<strong>en</strong>st Waterbeheer <strong>en</strong> Riolering),<br />

Paul Roeleveld later opgevolgd door Cora Uijterlinde (STOWA).<br />

Druk<br />

Kruyt Grafisch Advies bureau<br />

STOWA rapport 2002-43<br />

ISBN 90.5773.201.7


T<strong>en</strong> geleide<br />

Automatisering van process<strong>en</strong> staat volop in de belangstelling. Op steeds grotere schaal wordt<br />

binn<strong>en</strong> de waterschapp<strong>en</strong> nagedacht over de mogelijkhed<strong>en</strong> om process<strong>en</strong> te automatiser<strong>en</strong>,<br />

t<strong>en</strong>einde met de beperkte beschikbare ruimte te kunn<strong>en</strong> blijv<strong>en</strong> voldo<strong>en</strong> aan de steeds str<strong>en</strong>ger<br />

word<strong>en</strong>de efflu<strong>en</strong>teis<strong>en</strong> <strong>bij</strong> e<strong>en</strong> minimalisatie van de verwerkingskost<strong>en</strong>.<br />

Door gebrek aan goede <strong>en</strong> betrouwbare <strong>regelapparatuur</strong> voor <strong>slibontwatering</strong>sprocess<strong>en</strong> <strong>en</strong> de<br />

inhomog<strong>en</strong>iteit van het te ontwater<strong>en</strong> product slib, is e<strong>en</strong> vergaande automatisering van ontwateringsapparatuur<br />

tot op hed<strong>en</strong> nog niet gerealiseerd. De tot dusver bestaande regeling<strong>en</strong> blev<strong>en</strong><br />

beperkt tot e<strong>en</strong> geregelde vlokhulpmiddeldosering op basis van het slibdebiet <strong>en</strong>/of de slibconc<strong>en</strong>tratie.<br />

E<strong>en</strong> terugkoppeling met de ontwateringsapparatuur <strong>en</strong> de c<strong>en</strong>traat of filtraatkwaliteit<br />

werd vooralsnog niet gebruikt, terwijl dit e<strong>en</strong> noodzaak is voor de realisatie van e<strong>en</strong> regeling<br />

voor e<strong>en</strong> zelfoptimaliser<strong>en</strong>d ontwateringsproces.<br />

Het doel van het in dit rapport beschrev<strong>en</strong> STOWA onderzoek was dan ook het ontwerp<strong>en</strong> <strong>en</strong><br />

test<strong>en</strong> van e<strong>en</strong> regeling voor de automatische optimalisatie van het ontwateringsproces met<br />

voldo<strong>en</strong>de mogelijkhed<strong>en</strong> om variaties in conc<strong>en</strong>tratie <strong>en</strong> kwaliteit van het slib adequaat te<br />

verwerk<strong>en</strong>. Tev<strong>en</strong>s di<strong>en</strong>de de regeling geschikt te zijn voor meerdere types <strong>slibontwatering</strong>sapparatuur.<br />

Voorafgaand aan de ontwikkeling van de regeling zelf is eerst uitgebreid onderzoek gedaan<br />

naar bestaande meetapparatuur <strong>en</strong> regeling<strong>en</strong>. Dit literatuur- <strong>en</strong> praktijkonderzoek is in de<br />

periode 1996 tot <strong>en</strong> met 1999 uitgevoerd <strong>en</strong> is beschrev<strong>en</strong> in twee STOWA-rapport<strong>en</strong>, te wet<strong>en</strong>:<br />

<strong>Meet</strong>- <strong>en</strong> <strong>regelapparatuur</strong> <strong>bij</strong> <strong>slibontwatering</strong>:<br />

Inv<strong>en</strong>tarisatie van apparatuur <strong>en</strong> mogelijkhed<strong>en</strong>, STOWA 97-13<br />

<strong>Meet</strong>- <strong>en</strong> <strong>regelapparatuur</strong> <strong>bij</strong> <strong>slibontwatering</strong>:<br />

Vergelijk<strong>en</strong>d praktijkonderzoek aan meetapparatuur, STOWA 99-26<br />

Met dit derde <strong>en</strong> laatste rapport in de reeks ‘<strong>Meet</strong>- <strong>en</strong> <strong>regelapparatuur</strong> <strong>bij</strong> de <strong>slibontwatering</strong>’<br />

wordt het STOWA onderzoek afgeslot<strong>en</strong>.<br />

Het test<strong>en</strong> van de ontwikkelde regeling <strong>en</strong> de c<strong>en</strong>traatmeetopstelling was mogelijk door de<br />

goede sam<strong>en</strong>werking met de medewerkers van het Waterschap Zuiderzeeland (awzi Almere).<br />

De STOWA is deze medewerkers zeer erk<strong>en</strong>telijk voor hun waardevolle inbr<strong>en</strong>g.<br />

Het voorligg<strong>en</strong>de rapport beschrijft de ontwikkeling van e<strong>en</strong> regeling voor het automatiser<strong>en</strong><br />

van het ontwateringsproces <strong>en</strong> de resultat<strong>en</strong> die met de regeling zijn bereikt op de awzi Almere.<br />

Op deze locatie is aangetoond dat in combinatie met betrouwbare meetapparatuur het mogelijk<br />

is e<strong>en</strong> zelfoptimaliser<strong>en</strong>de ontwateringsinstallatie te realiser<strong>en</strong>. Met het uitgevoerde onderzoek<br />

is e<strong>en</strong> interessante ontwikkeling gepres<strong>en</strong>teerd, die zeker de moeite waard is om nader beschouwd<br />

te word<strong>en</strong> <strong>bij</strong> optimalisaties van het ontwateringsproces.<br />

Utrecht, januari 2003<br />

De directeur van de STOWA<br />

ir. J.M.J. Le<strong>en</strong><strong>en</strong>


SAMENVATTING<br />

Inleiding<br />

Reductie van de <strong>slibontwatering</strong>skost<strong>en</strong> is mogelijk door het optimaal bedrijv<strong>en</strong> van de slibonwateringsinstallatie<br />

gedur<strong>en</strong>de 24 uur per dag <strong>en</strong> 7 dag<strong>en</strong> per week met e<strong>en</strong> zelfoptimaliser<strong>en</strong>de<br />

regelunit.<br />

Uit e<strong>en</strong> in 1999 uitgevoerde b<strong>en</strong>chmarkstudie is geblek<strong>en</strong> dat 12 tot 45 % van de totale kost<strong>en</strong><br />

voor afvalwaterbehandeling nodig is voor de verwerking <strong>en</strong> afzet van slib. E<strong>en</strong> belangrijke maar<br />

relatief complexe stap in het verwerkingsproces is de <strong>slibontwatering</strong>. Variaties in slibkwaliteit<br />

mak<strong>en</strong> e<strong>en</strong> optimaal ontwateringsresultaat niet e<strong>en</strong>voudig. Voor e<strong>en</strong> storingsvrije bedrijfsvoering<br />

word<strong>en</strong> vooral in weekeind<strong>en</strong> <strong>en</strong> gedur<strong>en</strong>de de nacht “veilige” procesinstelling<strong>en</strong> gekoz<strong>en</strong>.<br />

E<strong>en</strong> zelfoptimaliser<strong>en</strong>d regelsysteem is in staat om ideale instelling<strong>en</strong> te handhav<strong>en</strong> <strong>en</strong> de<br />

instelling<strong>en</strong> van de ontwateringsinstallatie aan te pass<strong>en</strong> aan verander<strong>en</strong>de omstandighed<strong>en</strong>.<br />

Aangezi<strong>en</strong> er uit e<strong>en</strong> inv<strong>en</strong>tariser<strong>en</strong>de studie is geblek<strong>en</strong> dat er ge<strong>en</strong> goed werk<strong>en</strong>de regelsystem<strong>en</strong><br />

voorhand<strong>en</strong> war<strong>en</strong>, is in overleg met de STOWA beslot<strong>en</strong> e<strong>en</strong> regelsysteem te ontwerp<strong>en</strong><br />

<strong>en</strong> te test<strong>en</strong> in het kader van het STOWA onderzoek “Met<strong>en</strong> <strong>en</strong> regel<strong>en</strong> in de sliblijn”.<br />

In december 1999 is dit deel van het onderzoeksproject gestart met als primaire doel e<strong>en</strong><br />

regelsysteem te ontwerp<strong>en</strong> voor de optimalisatie van het ontwateringsproces, zodanig dat er<br />

voldo<strong>en</strong>de mogelijkhed<strong>en</strong> zijn om het systeem aan te pass<strong>en</strong> aan de verschill<strong>en</strong>de slibsoort<strong>en</strong> <strong>en</strong><br />

ontwateringsinstallaties.<br />

Opzet onderzoek<br />

Om e<strong>en</strong> goed beeld te krijg<strong>en</strong> waar e<strong>en</strong> regelunit aan moet voldo<strong>en</strong>, is voorafgaand aan het<br />

eig<strong>en</strong>lijke ontwerp van de regelunit eerst met e<strong>en</strong> afvaardiging van de Nederlandse<br />

waterkwaliteitsbeheerders gebrainstormd over de <strong>slibontwatering</strong> van de toekomst. Hieruit is<br />

naar vor<strong>en</strong> gekom<strong>en</strong> dat de huidige apparatuur op zich goed functioneert, maar dat e<strong>en</strong> regelunit<br />

nodig is om de wissel<strong>en</strong>de slibomstandighed<strong>en</strong> op e<strong>en</strong> rwzi goed te kunn<strong>en</strong> verwerk<strong>en</strong>. Met<br />

name het gebrek aan e<strong>en</strong> goed on-line inzicht in het ontwateringsresultaat maakt het moeilijk de<br />

apparatuur continu optimaal te lat<strong>en</strong> functioner<strong>en</strong> zonder toezicht. Aansluit<strong>en</strong>d zijn met e<strong>en</strong><br />

groep regeltechnici diverse regelconcept<strong>en</strong> geëvalueerd, waarna e<strong>en</strong> opzet is gemaakt voor de<br />

uiteindelijke regelunit.<br />

Sam<strong>en</strong> met Alfa Laval is de voorgestelde regelunit ontworp<strong>en</strong>, waar<strong>bij</strong> gebruik is gemaakt van<br />

drogestofs<strong>en</strong>sor<strong>en</strong> in het ingaande slib <strong>en</strong> in het c<strong>en</strong>traat. De keuze van drogestofs<strong>en</strong>sor<strong>en</strong> (Dr.<br />

Lange) is gebaseerd op de resultat<strong>en</strong> van e<strong>en</strong> eerder uitgevoerd onderzoek (STOWA 99-26).<br />

Resultat<strong>en</strong><br />

De regelunit is uitermate bedi<strong>en</strong>ingsvri<strong>en</strong>delijk opgezet. Met behulp van e<strong>en</strong> PC heeft de<br />

bedi<strong>en</strong>ingstechnicus/bedrijfsvoerder de mogelijkheid om e<strong>en</strong>voudig verschill<strong>en</strong>de randvoorwaard<strong>en</strong><br />

<strong>en</strong> procesinstelling<strong>en</strong> voor het <strong>slibontwatering</strong>sproces in te voer<strong>en</strong> (of te wijzig<strong>en</strong>). Zo<br />

word<strong>en</strong> minimale <strong>en</strong> maximale waard<strong>en</strong> opgegev<strong>en</strong> voor onder andere het slibdebiet, de PEdosering,<br />

het drogestofafscheidingsr<strong>en</strong>dem<strong>en</strong>t, het drogestofgehalte van het ontwaterde slib <strong>en</strong><br />

de instelling<strong>en</strong> van de ontwateringsapparatuur. Daarnaast word<strong>en</strong> alle relevante kost<strong>en</strong>factor<strong>en</strong><br />

ingevoerd, zoals de kost<strong>en</strong> voor PE, aanmaakwater, slibtransport, <strong>en</strong>ergie <strong>en</strong> slibafzet. Met de<br />

verschill<strong>en</strong>de machine-instelling<strong>en</strong> <strong>en</strong> de on-line meting van de conc<strong>en</strong>tratie van het te ontwater<strong>en</strong><br />

slib, wordt de ontwateringsinstallatie opgestart. Nadat deze e<strong>en</strong> bepaalde tijd op de setpoint<br />

instelling<strong>en</strong> heeft gedraaid, wordt automatisch overgeschakeld op e<strong>en</strong> optimalisatie programma.<br />

Afhankelijk van het gekoz<strong>en</strong> programma optimaliseert de regelunit het ontwateringsproces<br />

richting minimalisatie van kost<strong>en</strong> of naar e<strong>en</strong> zo hoog mogelijk drogestofgehalte van het<br />

ontwaterde slib.


De awzi Almere heeft als testlocatie gedi<strong>en</strong>d, waar<strong>bij</strong> één c<strong>en</strong>trifuge is uigevoerd met de<br />

regelunit. Aan de hand van test<strong>en</strong> onder verschill<strong>en</strong>de omstandighed<strong>en</strong> zijn de resultat<strong>en</strong> van<br />

beide c<strong>en</strong>trifuges vergelek<strong>en</strong>. De regelunit heeft voornamelijk in de kost<strong>en</strong>optimalisatiemodus<br />

gedraaid, waar<strong>bij</strong> de PE-dosering ca 25 % is gedaald <strong>en</strong> het einddrogestofgehalte op e<strong>en</strong><br />

constant niveau werd gehoud<strong>en</strong>. Drogestofvariaties in het ontwaterde slib over de dag, e<strong>en</strong><br />

verschijnsel dat voor de installatie van de regelunit regelmatig voorkwam, was na de installatie<br />

<strong>bij</strong>na geheel verdw<strong>en</strong><strong>en</strong>. Dit alles heeft erin geresulteerd dat met de regelunit ca. € 15,- per ton<br />

drogestof kan word<strong>en</strong> bespaard, wat voor de awzi Almere neerkomt op ruim € 53.000,- per jaar.<br />

Hier<strong>bij</strong> zijn nog ge<strong>en</strong> besparing<strong>en</strong> opg<strong>en</strong>om<strong>en</strong> voor minder toezicht <strong>en</strong> e<strong>en</strong> beter<br />

afscheidingsr<strong>en</strong>dem<strong>en</strong>t.<br />

Kost<strong>en</strong> <strong>en</strong> bat<strong>en</strong><br />

De kost<strong>en</strong> voor het installer<strong>en</strong> van e<strong>en</strong> regelunit zijn sterk afhankelijk van de lokale omstandighed<strong>en</strong><br />

op de rwzi. E<strong>en</strong> globale inschatting van de kost<strong>en</strong> voor hardware, software <strong>en</strong> de b<strong>en</strong>odigde<br />

tijd voor de installatie kom<strong>en</strong> uit op € 60.000,- tot € 105.000,- per ontwateringsunit.<br />

Ev<strong>en</strong>als de kost<strong>en</strong> zijn ook de bat<strong>en</strong> afhankelijk van de lokale situatie. Bov<strong>en</strong>di<strong>en</strong> zijn ze niet<br />

altijd gemakkelijk uit te drukk<strong>en</strong> in e<strong>en</strong> geldbedrag. De besparing<strong>en</strong> kunn<strong>en</strong> word<strong>en</strong> gerealiseerd<br />

doordat:<br />

de transport- <strong>en</strong> eindverwerkingskost<strong>en</strong> lager zull<strong>en</strong> uitvall<strong>en</strong> door e<strong>en</strong> constanter <strong>en</strong><br />

gemiddeld hoger drogestofgehalte van het ontwaterde slib;<br />

de personeelskost<strong>en</strong> omlaag gaan doordat de optimalisatie van het proces wordt overg<strong>en</strong>om<strong>en</strong><br />

door de regelunit;<br />

er minder PE- <strong>en</strong> PE-aanmaakwater wordt verbruikt;<br />

het afscheidingsr<strong>en</strong>dem<strong>en</strong>t van de installatie verbetert, zodat er minder slib opnieuw<br />

ontwaterd hoeft te word<strong>en</strong>;<br />

Conclusies<br />

Onderstaand zijn de voornaamste conclusies van het onderzoek opgesomd.<br />

Het is mogelijk om met e<strong>en</strong> regelunit het <strong>slibontwatering</strong>sproces continu te optimaliser<strong>en</strong>,<br />

zonder ingrijp<strong>en</strong> van e<strong>en</strong> operator gedur<strong>en</strong>de 24 uur per dag <strong>en</strong> 7 dag<strong>en</strong> per week;<br />

De opzet van de regelaar is universeel, maar vooralsnog is het regelconcept slechts uitgewerkt<br />

<strong>en</strong> getest met één type c<strong>en</strong>trifuge van één leverancier. Toepass<strong>en</strong> van de regelunit op<br />

andere c<strong>en</strong>trifuges of zeefbandpers<strong>en</strong> is in principe mogelijk, maar vraagt nog wel om e<strong>en</strong><br />

inspanning <strong>en</strong> investering van de leveranciers om het systeem te match<strong>en</strong> <strong>en</strong> de software<br />

aan te pass<strong>en</strong> voor de specifieke apparatuur;<br />

De daisywheel regeling <strong>en</strong> de fuzzy regeling moet<strong>en</strong> beide naast elkaar functioner<strong>en</strong>. De<br />

daisywheel regeling is nodig voor de fijnafstemming, maar is ongeschikt om snelle wisseling<strong>en</strong><br />

te realiser<strong>en</strong>. Hier is e<strong>en</strong> fuzzy regelaar voor nodig. Voor relatief stabiele slibkwaliteit<strong>en</strong><br />

<strong>en</strong> geringe drogestofconc<strong>en</strong>tratiewisseling<strong>en</strong> kan (ev<strong>en</strong>tueel) word<strong>en</strong> volstaan met<br />

<strong>en</strong>kel e<strong>en</strong> daisywheel regeling.<br />

Voor het implem<strong>en</strong>ter<strong>en</strong> van e<strong>en</strong> regelunit, zoals is ontwikkeld, moet er e<strong>en</strong> zekere relatie<br />

zijn tuss<strong>en</strong> het drogestofgehalte van het ontwaterde slib <strong>en</strong> de PE-dosering <strong>en</strong>/of de weerstand<br />

in de c<strong>en</strong>trifuge. Deze relaties moet<strong>en</strong> word<strong>en</strong> uitgezocht, zodat de regelunit kan<br />

word<strong>en</strong> geijkt aan de werkelijkheid;<br />

De test<strong>en</strong> in Almere zijn uitgevoerd met e<strong>en</strong> lastig te ontwater<strong>en</strong> aëroob gestabiliseerd slib.<br />

Met anaëroob gestabiliseerd slib is er meestal e<strong>en</strong> betere relatie te vind<strong>en</strong> tuss<strong>en</strong> de ontwateringsparameters<br />

<strong>en</strong> is dus te verwacht<strong>en</strong> dat met de inzet van de regelunit ook hier goede<br />

resultat<strong>en</strong> te behal<strong>en</strong> zijn;<br />

De mogelijke besparing<strong>en</strong> hang<strong>en</strong> sterk af van de lokale omstandighed<strong>en</strong>, <strong>bij</strong>voorbeeld of<br />

de verrek<strong>en</strong>ing van ontwaterd slib <strong>bij</strong> de eindverwerker/afnemer op basis van drogestof of<br />

aangeleverde tonn<strong>en</strong> slib plaatsvindt;


E<strong>en</strong> belangrijke randvoorwaarde voor e<strong>en</strong> goede werking van de regelunit is e<strong>en</strong> betrouwbare<br />

drogestofmeter in het ingaande slib <strong>en</strong> het c<strong>en</strong>traat of filtraat, waar<strong>bij</strong> de opstelling in<br />

het filtraat of c<strong>en</strong>traat uitermate belangrijk is;<br />

De Dr. Lange s<strong>en</strong>sor<strong>en</strong> hebb<strong>en</strong> gedur<strong>en</strong>de de gehele testperiode van meer dan e<strong>en</strong> jaar<br />

uitstek<strong>en</strong>d gefunctioneerd. Storing<strong>en</strong> <strong>bij</strong> meting van het ingaande slib hebb<strong>en</strong> niet plaatsgevond<strong>en</strong>.<br />

De variaties in kwaliteit van het ingaande slib over het jaar kond<strong>en</strong> e<strong>en</strong>voudig<br />

word<strong>en</strong> <strong>bij</strong>gesteld door variatie van de scalingsfactor <strong>en</strong> het onderhoud bleef beperkt tot<br />

het vervang<strong>en</strong> van e<strong>en</strong> wisser;<br />

De keuze van het PE di<strong>en</strong>t kritisch te word<strong>en</strong> geanalyseerd, omdat de gevoeligheid <strong>bij</strong><br />

overdosering erg stor<strong>en</strong>d is voor e<strong>en</strong> optimale regeling;<br />

Eén van de nadel<strong>en</strong> is de complexiteit van de regelunit <strong>en</strong> de daarmee sam<strong>en</strong>hang<strong>en</strong>de hoge<br />

prijs. Hierteg<strong>en</strong>over staan echter de nodige besparing<strong>en</strong>, zodat de regelunit in principe in <strong>en</strong>kele<br />

jar<strong>en</strong> terugverdi<strong>en</strong>d moet kunn<strong>en</strong> word<strong>en</strong>.


De STOWA in het kort<br />

De Stichting Toegepast Onderzoek Waterbeheer, kortweg STOWA, is het onderzoeksplatform<br />

van Nederlandse waterbeheerders. Deelnemers zijn alle beheerders van grondwater <strong>en</strong> oppervlaktewater<br />

in landelijk <strong>en</strong> stedelijk gebied, beheerders van installaties voor de zuivering van<br />

huishoudelijk afvalwater <strong>en</strong> beheerders van waterkering<strong>en</strong>. In 2002 war<strong>en</strong> dat alle waterschapp<strong>en</strong>,<br />

hoogheemraadschapp<strong>en</strong> <strong>en</strong> zuiveringsschapp<strong>en</strong>, de provincies <strong>en</strong> het Rijk (i.c. het Rijksinstituut<br />

voor Zoetwaterbeheer <strong>en</strong> de Di<strong>en</strong>st Weg- <strong>en</strong> Waterbouw).<br />

De waterbeheerders gebruik<strong>en</strong> de STOWA voor het realiser<strong>en</strong> van toegepast technisch, natuurwet<strong>en</strong>schappelijk,<br />

bestuurlijk juridisch <strong>en</strong> sociaal-wet<strong>en</strong>schappelijk onderzoek dat voor h<strong>en</strong> van<br />

geme<strong>en</strong>schappelijk belang is. Onderzoeksprogramma’s kom<strong>en</strong> tot stand op basis van behoefteinv<strong>en</strong>tarisaties<br />

<strong>bij</strong> de deelnemers. Onderzoekssuggesties van derd<strong>en</strong>, zoals k<strong>en</strong>nisinstitut<strong>en</strong> <strong>en</strong><br />

adviesbureaus, zijn van harte welkom. Deze suggesties toetst de STOWA aan de behoeft<strong>en</strong> van<br />

de deelnemers.<br />

De STOWA verricht zelf ge<strong>en</strong> onderzoek, maar laat dit uitvoer<strong>en</strong> door gespecialiseerde<br />

instanties. De onderzoek<strong>en</strong> word<strong>en</strong> begeleid door begeleidingscommissies. Deze zijn sam<strong>en</strong>gesteld<br />

uit medewerkers van de deelnemers, zonodig aangevuld met andere deskundig<strong>en</strong>.<br />

Het geld voor onderzoek, ontwikkeling, informatie <strong>en</strong> di<strong>en</strong>st<strong>en</strong> br<strong>en</strong>g<strong>en</strong> de deelnemers sam<strong>en</strong><br />

<strong>bij</strong>e<strong>en</strong>. Mom<strong>en</strong>teel bedraagt het jaarlijkse budget zo’n vijf miljo<strong>en</strong> euro.<br />

U kunt de STOWA bereik<strong>en</strong> op telefoonnummer: +31 (0)30-2321199.<br />

Ons adres luidt: STOWA, Postbus 8090, 3503 RB Utrecht.<br />

Email: stowa@stowa.nl.<br />

Website: www.stowa.nl.


SUMMERY<br />

Introduction<br />

The application of a self-optimising control-unit for the operation of a sludge dewatering<br />

facility, during tw<strong>en</strong>ty-four hours per day and sev<strong>en</strong> days per week, will decrease the operation<br />

costs.<br />

A b<strong>en</strong>chmark study of 1999 has shown that the costs for treatm<strong>en</strong>t of excess sludge and transport<br />

of treated sludge amounts to 12 – 45% of the total costs for sewage treatm<strong>en</strong>t. The sludge<br />

dewatering is an important, but relatively complex step in the treatm<strong>en</strong>t process. Because of<br />

variations in the quality of the sludge, it is not an easy task to obtain an optimal result of<br />

dewatering. To prev<strong>en</strong>t operational upsets during nights and week<strong>en</strong>ds “safe” process parameters<br />

are used, which do not reflect anymore than the approximate average parameters.<br />

A self-optimising control-unit is able to follow process changes and maintain the best operational<br />

parameters throughout the operation of the sludge dewatering facility.<br />

An inv<strong>en</strong>tory on the availability of control-units has learned that no suitable control-unit was<br />

available, h<strong>en</strong>ce has be<strong>en</strong> decided in close collaboration with STOWA to design and test a<br />

suitable control-system within the framework of the STOWA project “Measuring and controlling<br />

sludge dewatering processes”.<br />

In December 1999 this part of the investigations has embarked with the main objective to<br />

design a suitable control-system for the optimisation of the sludge dewatering process. The<br />

control-system should have suffici<strong>en</strong>t possibilities to be adapted to the several types of sludge<br />

and sludge dewatering facilities.<br />

Methodology<br />

To obtain a good picture of the specifications a control-unit should meet, a preliminary brainstorm<br />

session was held, aimed at the sludge dewatering facility of the future. A delegation of<br />

the water authorities was invited to participate in the aforem<strong>en</strong>tioned brainstorm session.<br />

The main conclusion of the brainstorm session was that the existing sludge dewatering facilities<br />

are performing well, but a control-unit is needed to cope with the varying sludge conditions that<br />

occur at sewage treatm<strong>en</strong>t plants; especially the lack of a clear, on-line, insight in the results of<br />

the sludge dewatering process make it difficult to operate such a system without supervision on<br />

a continuous basis.<br />

Subsequ<strong>en</strong>tly, several differ<strong>en</strong>t control concepts have be<strong>en</strong> evaluated by a group of process<br />

control specialists, after which a proposal has be<strong>en</strong> made for the design of the selected controlunit.<br />

Together with Alfa Laval the proposed control-unit has be<strong>en</strong> designed, making use of dry-solids<br />

s<strong>en</strong>sors in the sludge feed and in the c<strong>en</strong>trate. The selection of dry-solids s<strong>en</strong>sors (brand: Dr.<br />

Lange) has be<strong>en</strong> based on the results of an earlier investigation (STOWA 99-26).<br />

Results<br />

The set-up of the control-unit is exceedingly user-fri<strong>en</strong>dly. By means of a microcomputer the<br />

operator/supervisor has the possibility to input or change simply several conditions and process<br />

adjustm<strong>en</strong>ts for the sludge dewatering process. Minimum and maximum values are put in for<br />

(among other things): Sludge flow rate, PE-dosing, dry-solids recovery, dry-solids cont<strong>en</strong>t of<br />

the dewatered sludge and the operational parameters of the sludge dewatering equipm<strong>en</strong>t. Next,<br />

all relevant cost-factors are put in, such as the costs for the PE, make-up water, sludge transport,<br />

<strong>en</strong>ergy and cost factors for further sludge treatm<strong>en</strong>t and/or disposal.<br />

The sludge dewatering facility is started-up with the input of the several operational parameters<br />

and the on-line measurem<strong>en</strong>t of the dry-solids cont<strong>en</strong>t in the sludge feed. After running a certain


time with the operational parameters at set point, the control-unit automatically switches to an<br />

optimisation program. Dep<strong>en</strong>ding of the selected program the control-unit optimises the sludge<br />

dewatering process either towards minimisation of the costs or towards a maximisation of the<br />

dry-solids cont<strong>en</strong>t of the dewatered sludge.<br />

The sewage treatm<strong>en</strong>t plant in Almere served as a location for testing of the control-unit, where<br />

one decanter c<strong>en</strong>trifuge has be<strong>en</strong> equipped with the control-unit and the other one without.<br />

Following a testing program under differ<strong>en</strong>t process conditions the results of both c<strong>en</strong>trifuges<br />

have be<strong>en</strong> compared. The control-unit has be<strong>en</strong> operating predominantly in the costoptimisation<br />

mode, where the PE-dosing has be<strong>en</strong> decreased 25% and the dry-solids cont<strong>en</strong>t of<br />

the dewatered sludge has be<strong>en</strong> kept on a constant level. Variations in the dry-solids cont<strong>en</strong>t of<br />

the dewatered sludge occurred quite regularly before the installation of the control-unit. These<br />

variations had disappeared after the installation of the control-unit. All this has resulted in a cost<br />

b<strong>en</strong>efit of € 15.- per ton dry-solids, which boils down for the sewage treatm<strong>en</strong>t plant Almere to<br />

a cost b<strong>en</strong>efit of more than € 53,000.- per year, without the extra cost b<strong>en</strong>efit due to less supervision<br />

and an improved dry-solids recovery.<br />

Costs and B<strong>en</strong>efits<br />

The costs for the installation of the self-optimising control-unit dep<strong>en</strong>d strongly on the local<br />

conditions of the sewage treatm<strong>en</strong>t plant. An approximation of the costs involved for hardware,<br />

software and the installation would range from € 60,000.- to € 105,000.-.<br />

In analogy the b<strong>en</strong>efits dep<strong>en</strong>d as strongly on the local conditions. Furthermore it is not always<br />

simple to express the b<strong>en</strong>efits in an exact amount of money. The b<strong>en</strong>efits can be realised,<br />

because:<br />

the costs of transport and the final treatm<strong>en</strong>t costs will decrease, due to a more constant<br />

and higher average dry-solids cont<strong>en</strong>t of the dewatered sludge;<br />

the labour costs decrease, due to the optimisation of the dewatering process, tak<strong>en</strong> over by<br />

the control-unit;<br />

a decreased usage of PE and PE make-up water;<br />

an improvem<strong>en</strong>t of the dry-solids recovery, resulting in less sludge to dewater over again.<br />

Conclusions<br />

Below is a list of the most important conclusions of this investigation.<br />

It is possible to optimise the sludge dewatering process by means of a control-unit, without<br />

the interv<strong>en</strong>tion of process operators, during tw<strong>en</strong>ty-four hours per day and sev<strong>en</strong> days per<br />

week.<br />

In principal the set-up of the control-unit is universal, but the control concept has only be<strong>en</strong><br />

tested with one type of c<strong>en</strong>trifuge from one supplier. Usage of the control-unit for other<br />

c<strong>en</strong>trifuges or belt filter presses should be possible, but needs an additional investm<strong>en</strong>t and<br />

effort of the suppliers to match the control-unit to the specific dewatering system.<br />

The daisywheel concept is specially made for the fine tuning of the dewatering process in<br />

contrast to the fuzzy logic concept, which is specially made for fast changing conditions.<br />

Both should be incorporated in the control-unit to be sure that the control unit is always<br />

able to optimise the dewatering process, ev<strong>en</strong> wh<strong>en</strong> the optimisation process starts far<br />

away from the optimal dewatering settings. Wh<strong>en</strong> the incoming sludge doesn’t vary a lot in<br />

quality it is possible to optimise the process only with the daisywheel concept.<br />

For the implem<strong>en</strong>tation of this control-unit in particular, there should be a distinct correlation<br />

betwe<strong>en</strong> the dry-solids cont<strong>en</strong>t of the dewatered sludge and/or the friction in the c<strong>en</strong>trifuge.<br />

These correlations have to be established to <strong>en</strong>able a realistic calibration of the<br />

control-unit in practice.<br />

The tests in Almere have be<strong>en</strong> conducted using an aerobically stabilised sludge, which is<br />

difficult to dewater. In g<strong>en</strong>eral better results with the control-unit can be expected wh<strong>en</strong> anaerobically<br />

stabilised sludge is involved, due to the better dewatering properties of this<br />

sludge.


The feasible b<strong>en</strong>efits are strongly dep<strong>en</strong>ding on local conditions. For example, the differ<strong>en</strong>ce<br />

in calculating costs for further sludge treatm<strong>en</strong>t may be based on tons of sludge or<br />

based on dry-solids cont<strong>en</strong>t, which dep<strong>en</strong>ds on the owner of the installation.<br />

An important condition to assure correct operation of the control-unit is the application of a<br />

reliable dry-solids measurem<strong>en</strong>t in the feed sludge and the c<strong>en</strong>trate or filtrate, where the installation<br />

in the c<strong>en</strong>trate or filtrate is of the utmost importance.<br />

The s<strong>en</strong>sors of Dr. Lange have operated correctly during the <strong>en</strong>tire test period of more than<br />

a year. No interfer<strong>en</strong>ces of the dry-solids measurem<strong>en</strong>t in the feed sludge have occurred.<br />

The variations in the quality of the feed sludge could simply be adjusted by means of the<br />

variation of the scaling factor. Maint<strong>en</strong>ance kept restricted to the replacem<strong>en</strong>t of a wiper.<br />

The correct selection of the PE is very important, because of the s<strong>en</strong>sitivity of the controlunit<br />

for overdosing.<br />

A main disadvantage is the complexity of the control-unit causing the high price. However, the<br />

b<strong>en</strong>efits will <strong>en</strong>able payback times of a few years.


STOWA in brief<br />

The Institute of Applied Water Research (in short, STOWA) is a research platform for Dutch<br />

water controllers. STOWA participants are ground and surface water managers in rural and<br />

urban areas, managers of domestic wastewater purification installations and dam inspectors. In<br />

2002 that includes all the country’s water boards, polder and dike districts and water treatm<strong>en</strong>t<br />

plants, the provinces and the State<br />

These water controllers avail themselves of STOWA’s facilities for the realisation of all kinds of<br />

applied technological, sci<strong>en</strong>tific, administrative-legal and social-sci<strong>en</strong>tific research activities<br />

that may be of communal importance. Research programmes are developed on the basis of<br />

requirem<strong>en</strong>t reports g<strong>en</strong>erated by the institute’s participants. Research suggestions proposed by<br />

third parties such as c<strong>en</strong>tres of learning and consultancy bureaux, are more than welcome.<br />

After having received such suggestions STOWA th<strong>en</strong> consults its participants in order to verify<br />

the need for such proposed research.<br />

STOWA does not conduct any research itself, instead it commissions specialised bodies to do<br />

the required research. All the studies are supervised by supervisory boards composed of staff<br />

from the various participating organisations and, where necessary, experts are brought in.<br />

All the money required for research, developm<strong>en</strong>t, information and other services is raised by<br />

the various participating parties. At the mom<strong>en</strong>t, this amounts to an annual budget of some five<br />

million euro.<br />

For telephone contact STOWA’s number is: (31 (0)30-2321199.<br />

The postal address is: STOWA, P.O. Box 8090, 3503 RB, Utrecht.<br />

E-mail: stowa@stowa.nl.<br />

Website: www.stowa.nl.


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