Thermal and hydraulic modeling of an anaerobic digestion plant

Vermaak, Rynhardt Petrus (2016-03)

Thesis (MEng)--Stellenbosch University 2016.

Thesis

ENGLISH ABSTRACT: The use of an anaerobic digestion (AD) process to treat municipal sewage sludge is one of the few options that provides the ability to treat sludge, offers a sustainable option for a carbon-neutral energy source and ability to reclaim wastewater back to potable water. The importance of the anaerobic digestion plant layout is investigated in terms of its process capacity, energy production and electrical energy generation based on thermal numerical modelling. A design is proposed in the study that is able to treat 986.742 m3/day at 4%TS (81 %) of the available sludge at the wastewater treatment plant, generating 1.293 MW net electrical power and still meet the heat requirements of the anaerobic digestion plant as well as the quality of effluent discharged by the plant. The proposed design is able to treat 53 % more sludge and generate 45.4 % more electrical net power compared to the first conceptual plant layout using a 3/2 two-stage digester configuration. With the use of hydraulic numerical modelling and experimental work, a mixing system was developed and proposed, that satisfies the biological requirement of micro bacteria inside the digester. The benefits of selecting the proposed mixing system over the mechanical draft tube method include: all the mechanical equipment of the mixing system is located outside the digester, it has no moving parts inside the digester, tedious downtime of clogged rotating equipment is eliminated and a reduction in biogas production rate when the digester is opened and exposed to air for maintenance can be eliminated. The proposed mixing system consists of five 518 mm draft tubes using a two-phase plug-flow regime formed by five plug-flow generators placed at the bottom of the draft tubes. The mixing system proposed was able to achieve a digester volume turn over time (DVTT) from 0.82 to 1.3 h, at a mixing intensity (G) from 70.58 to 35.88 s-1 requiring a unit power per volume (UP) from 3.41 to 0.88 W/m3. Stellenbosch University https://scholar.sun.ac.za

AFRKAANSE OPSOMMING: Anaërobiese verteer (AV)-prosesse is een van min behandelingsmetodes wat gebruik kan word om munisipale rioolslyk te behandel en bied, ’n volhoubare moontlikheid van ’n koolstofneutrale energiebron, en die vermoë inhou om afvalwater tot drinkbare water te suiwer. Die belangrikheid van die AV-aanlegontwerp is ondersoek aan die hand van werkverrigting in die stabilisering van slyk hoeveelheid sowel as, elektriese en energieproduksie wat gebaseer was op grond van ’n termiese numeriese model. Die voorgestelde ontwerp van die gevallestudie kon daarin slaag om 986.742 m3/dag teen 4%TS (81 %) van die beskikbare slyk by die afvalwatersuiweringsaanleg te behandel, 1.293 MW netto elektriese krag opwek en steeds aan die hitte vereiste van die aanleg voldoen asook die kwaliteit van wat afgevoer water deur die aanleg. Die voorgestelde ontwerp kon 53% meet slyk hanteer en kon 45.4 % meer elektriese genereer vergeleke met die eerste voorgestelde ontwerp wat gebruik maak van n 3/2 twee-fase verteerontwerp. Hidrouliese numeriese modellering en eksperimentele werk is gebruik om ’n mengstelsel ontwikkel en voorgestel wat aan die biologiese vereiste van mikrobakterieë binne-in die toestel voldoen. Die voordele van die gebruik van die voorgestelde mengstelsel bo die van die ‘n meganiese afleibuise ontwerp metode sluit die volgende in: al die meganiese toerusting word buite die verteerstelsel gemonteer, dit het geen bewegende dele in die verteerstelsel, die vermorsing van tyd deur rotering toerusting skoon te maak asook die vermindering van die produsering van biogas as gevolg van die oopmaak die verteerstelsel wat die verteerstelsel blootstel aan lug kan vermy word. Die mengstelsel bestaan uit vyf afleibuise van 518 mm, wat gebruik maak van die tweefase-propvloei form wat deur ’n propvloei-opwekker voortgebring word om die verteertoestel te meng. Die mengstelsel kon ‘n verteerstelsel volume omkeertyd van 0.82 tot 1.3 h behaal, teen ‘n meng intensiteit (G) van 70.58 tot 35.88 s-1 wat 'n eenheid krag per volume vereis van (UP) 3.41 tot 0.88 W/m3.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98816
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