Plassen tegen de stank: hoe jouw urine onze riolering kan redden
Iedereen moet plassen, maar toch staan we zelden stil bij water gebeurt wanneer onze plas door het toilet verdwijnt. Voor de meeste mensen is urine niet meer dan een onderdeel van het afvalwater. Maar wat als ik zeg dat we jouw urine naar een waardevolle grondstof kunnen omzetten? Dit goedje kan een sleutel zijn in de strijd tegen stinkende riolen, beschadigingen aan de rioolinfrastructuur én vervuilende emissies. Ook kan het bijdragen als oplossing voorde grote stikstofbelasting op onze waterzuiveringsinstallaties.
Een riool vol uitdagingen
Je staat er misschien niet bij stil, maar in onze riolering vinden er allerlei microbiële processen plaats. Sommige zijn nuttig voor ons, andere schadelijk. Een van de grootste boosdoeners is waterstofsulfide (H2S). Dit gas ontstaat onder zuurstofarme omstandigheden en zorgt voor de stinkende rioolgeur en aantasting van de pijpleidingen. Vandaag pakken we deze problemen vooral chemisch aan, met toevoeging van nitraat of ijzerzout. Dit werkt, maar de chemische stoffen zijn duur en niet-hernieuwbaar. Wat als we slimmer konden omgaan met wat we al in overvloed hebben? Stel je nu eens voor: minder stank in de straat én minder dure rioolwerken zonder dat daar (dure) chemicaliën voor nodig zijn. Dit kan, dankzij urine!
Urine, een gouden kans!
Urine bevat van nature veel stikstof, die via biologische voorbehandeling (nitrificatie) kan worden omgezet in nitraat (een vorm van stikstof). Dit nitraat kan vervolgens op een slimme manier in de riolering gedoseerd worden om schadelijke processen te onderdrukken en nuttige bacteriën te stimuleren. Op deze manier wordt urine een troef voor duurzaamheid. Zo verandert een alledaags afvalproduct in een circulaire grondstof die niet alleen stank en corrosie vermindert, maar ook de levensduur van riolen verlengt, de stikstofdruk op zuiveringsinstallaties verlaagt en de afhankelijkheid van niet-hernieuwbare chemicaliën vermindert.
Maar hoe werkt dat precies? In een riool zonder zuurstof, maar in aanwezigheid van nitraat nemen deze twee belangrijke bacteriegroepen het over:
- Heterotrofe denitrificeerders: Deze gebruiken organische stoffen en zetten nitraat of genitrificeerde urine om naar stikstofgas. Dit proces is snel en efficiënt, en kan helpen de stikstofdruk op waterzuiveringsinstallaties en in het milieu te verlagen.
-
Sulfide-gedreven autotrofe denitrificeerders: Deze zetten ook nitraat om naar stikstofgas, maar in plaats van organische stoffen gebruiken ze sulfide (H2S) als energiebron, waardoor de stank en schade aan de riolering verlaagt.
De potentiële voordelen zijn aangegeven met groene pijlen. Ook worden de twee bacteriële processen weergegeven: sulfide-gedreven autotrofe denitrificatie (SAD) en heterotrofe denitrificatie (HD). De rode pijl geeft de inhibitie van HD door sulfide aan.
Deze twee groepen spelen de hoofdrol in mijn onderzoek. In theorie zouden beide groepen elkaar kunnen helpen om de stikstofverwijdering in de riolering te vergemakkelijken. Maar de mening hierover in de literatuur is verdeeld. Sommige studies rapporteren synergie (samenwerking met een versterkend effect), andere juist competitie. Ook is bekend dat H2S giftig is voor de heterotrofen, maar de hoeveelheid H2S die nodig is om een toxisch effect te bekomen is niet eenduidig.
Daarom wou ik drie dingen onderzoeken:
- Hoe snel kunnen heterotrofe denitrificeerders en sulfide-gedreven autotrofe denitrificeerders nitraat verwijderen?
- Wat gebeurt er als beide groepen tegelijk aanwezig zijn?
- Welke hoeveelheid H2S is toxisch voor heterotrofe denitrificatie?
- Is jouw urine voldoende om het nitraatniveau in de riolering voldoende hoog te houden?
Proefopzet
Om dit te onderzoeken werkte ik met activiteitstesten. Dit zijn testen waarbij de activiteit van bacteriën wordt opgevolgd in kleine flessen onder gecontroleerde omstandigheden: welke stoffen
aanwezig zijn en, in welke hoeveelheden en welke bacteriën hun werk doen. Ik voerde drie soorten testen uit: 1) heterotrofe testen, met acetaat (een organische koolstofbron) als energiebron. 2) autotrofe testen met H₂S, en 3) gemengde testen met zowel acetaat als H₂S.
Tijdens deze testen mat ik hoe snel nitraat verdween, hoeveel sulfide omgezet werd en of het proces geremd werd door bepaalde stoffen. Voor de autotrofe testen moest ik het slib dat ik verkreeg van een echte waterzuiveringsinstallatie op voorhand voeden, ook wel verrijken genoemd, met H₂S, om de autotrofen op te kweken.
Bacteriën in de ring: Het gevecht om nitraat
Wat bleek? Heterotrofe denitrificatie is veruit de snelste: Hun activiteit was zes keer hoger dan sulfide-gedreven autotrofe denitrificatie in het begin. Sulfide-gedreven autotrofe denitrificatie startte traag, maar na verrijking verdubbelde hun activiteit bijna. Wanneer beide samen actief waren, bleek er geen samenwerking te zijn: Heterotrofe denitrificatie kreeg de bovenhand zolang er organische koolstof aanwezig was. Verder zorgde de invloed van H₂S op heterotrofe denitrificatie duidelijk voor remming.
De laatste onderzoeksvraag beantwoordde ik met een computermodel dat berekende hoeveel nitraat nodig is om de gewenste omstandigheden in stand te houden. Daaruit bleek dat voorbehandelde urine op zichzelf genoeg nitraat kan leveren om stank en corrosie in toom te houden, zonder dat er externe chemicaliën nodig zijn.
De toekomst door een porseleinen bril: Het belang van het onderzoek
Mijn onderzoek is belangrijk om drie redenen. 1) Het omvat een standaardmethode voor sulfide-gedreven autotrofe denitrificatie activiteit, die reproduceerbare testen mogelijk maakt. 2) Het geeft inzichten in de interacties tussen beide groepen, waarbij blijkt dat heterotrofe denitrificatie meestal domineert zolang er organische koolstof aanwezig is. 3) Een praktische vertaalslag, door laboresultaten te koppelen aan simulaties die aangeven hoeveel urine er precies nodig is.
De toekomst van onze riolen zou wel eens in ons toilet kunnen liggen. Wat vandaag wordt doorgespoeld, kan morgen het verschil maken tussen stinkende, corroderende leidingen en een duurzaam, circulair watersysteem. Urine is dus geen afval, maar een onverwachte bondgenoot in de strijd tegen stank, vervuiling en verspilling. Misschien wordt het dus tijd om jouw urine nuttig in te zetten als een kleine dagelijkse bijdrage aan een schonere en duurzamere waterzuivering!
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