Paper industry process wastewater reclamation and potential clarification from paper sludge through integrated bio-energy production

Donkor, Kwame Ohene (2019-04)

Thesis (MEng)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: The paper and pulp industry is one of the major consumers of fresh water and as such produces large quantities of contaminated process water. However, with the recent drought crisis in South Africa, there has been a growing need amongst the paper and pulp community to reduce their water footprint. One potential strategy is to reclaim water from the paper waste sludge. Paper waste sludge (PS) consists of high amounts of cellulose and ash, with about 50 to 80% moisture content. Bioprocessing methods such as fermentation and anaerobic digestion with clean water have been reported to convert paper sludge into bioenergy thereby avoiding the urge of establishing a close water loop system. Also very little information on the potential of bioprocessing technologies to recover entrapped water molecules in paper sludge have been reported. In this study, a sequential fermentation and anaerobic digestion model using process water (COD > 2000 mg/L) as make up stream was explored to ascertain the potential of water reclamation from paper sludge while simultaneously producing bioenergy. Three paper waste sludges, i.e. virgin pulp, corrugated recycle and tissue printed recycle with their corresponding process water samples were utilized in this study. All the sludges and their process waters were obtained from the primary clarifiers of pulp mills. Fermentation and anaerobic digestion performances in terms of energy production were the same when using clean water and recycled process water in screening experiments. Paper sludge conversion to ethanol by fermentation, as performed in bioreactors, could reclaim in excess of 80% of the water present in the solids initially, but simultaneously increased the COD of the reclaimed process water from 4 780 mg/L to 86 800 mg/L. Alternatively, anaerobic digestion applied to similar paper sludge and process water samples could reclaim about 50% of water from paper sludge solids, and achieved a 20% to 40% reduction of COD in reclaimed process water. The proposed model of sequential bioprocessing of paper waste sludge through fermentation and anaerobic digestion achieved water reclamation similar to that obtained by the fermentation process but also increased the process water COD from 4 780 mg/L to 72 500 mg/L. In addition to water reclamation, the sequential bioprocessing of paper sludge produced about 20% to 60% more bioenergy than the fermentation or anaerobic digestion could achieve by themselves. Fermentation accounted for about 50% to 80% of the bioenergy produced in the combined process; for example, fermentation of virgin pulp paper sludge gave the highest ethanol yield of 275.4 kg ethanol/ton dry PS; which accounted for 80% of the total product energy (10 650 MJ/kg ton PS). Although corrugated recycle produced a lower ethanol yield (152.2 kg ethanol/ton dry PS) as compared to virgin pulp, the fermentation residues were better suited for anaerobic digestion, which contributed 50% of the total product energy (9 288 MJ/kg ton PS). Moreover, anaerobic digestion of fermented stillage had the added benefit of a short (5 to 10-days) biogas production period. In conclusion, sequential biochemical processing of paper sludge as compared to individual processes was better in maximizing both bio-energy and water reclamation. Alternatively, the sequential process considerably worsened the COD of the reclaimed water. Consequently, the water reclaimed is not immediately reusable without further wastewater treatment. The sequential approach was also able to significantly reduce the amount of solid waste which also showed promising applications in the agricultural and industrial sector.

AFRIKAANSE OPSOMMING: Die papier- en pulpindustrie is een van die grootste verbruikers van vars water en produseer as sulks groot hoeveelhede gekontamineerde proseswater. Met die onlangse droogtekrisis in Suid-Afrika, is daar egter ʼn groeiende behoefte in die papier- en pulpgemeenskap om hul watervoetspoor te verminder. Een potensiële strategie is om water uit die paperafvalslyk te herwin. Paperafvalslyk (PS) bestaan uit hoë hoeveelhede sellulose en as, met omtrent 50% tot 80% voginhoud. Bioprosesseringmetodes soos fermentasie en anaerobiese vertering met skoon water is berig om paperslyk in bio-energie om te kan skakel, wat daardeur die behoefte vir ʼn geslote lus waterstelsel vermy. Daar is ook baie min informasie oor die potensiaal van bioprosseseringtegnologië om vasgevange watermolekules in papierslyk te herwin. In hierdie studie is ʼn sekwensiële fermentasie en anaerobiese vertering model wat proseswater (COD > 2000 mg/L) as aanvullingsstroom ondersoek om die potensiaal van waterherwinning uit papierslyk vas te stel terwyl bio-energie gelyktydig vervaardig word. Drie papierafvalslyke, i.e. nuutpulp, geriffelde herwinning en tissue-gedrukte herwinning met hul ooreenstemmende proefsteke van proseswater, is gebruik in hierdie studie. Beide die slyke en hul proseswater is verkry deur die primêre verhelderaar van pulpmeule. Fermentasie en anaerobiese vertering doeltreffendheid in terme van energie produksie was dieselfde toe skoon water en herwinde proseswater in siftingseksperimente gebruik is. Papierslykomsetting na etanol by fermentasie, soos gebruik in bioreaktors, kon aanvanklik ʼn oormaat van 80% van die water teenwoordig in vastestowwe herwin, maar het gelyktydig die COD van die herwinde proseswater van 4 780 mg/L na 86 800 mg/L verhoog. Alternatiewelik het anaerobiese vertering toegepas op soortgelyke slyk en proseswaterproefsteke omtrent 50% van water uit papierslyk vastestowwe herwin, en ʼn 20% tot 40% vermindering van COD in herwinde proseswater bereik. Die voorgestelde model van sekwensiële bioprosessering van papierafvalslyk deur fermentasie en anaerobiese vertering het waterherwinning bereik soortgelyk aan dié verkry deur die fermentasieproses maar het ook die proseswater COD van 4 780 mg/L na 72 500 mg/L verhoog. Buiten waterherwinning het die sekwensiële bioprosesering van papierslyk omtrent 20% tot 60% meer bioenergie vervaardig as wat die fermentasie of anaerobiese verteerder op hul eie kon bereik. Fermentasie was verantwoordelik vir omtrent 50% tot 80% van die bio-energie vervaardig in die gekombineerde proses. Byvoorbeeld, fermentasie van nuutpulppapierslyk het die hoogste etanol opbrengs van 275.4 kg etanol/ton droeë PS gegee, wat rekenskap gee vir 80% van die totale produkenergie (10 650 MJ/kg ton PS). Alhoewel geriffelde herwinning ʼn laer etanol opbrengs gegee het (152.2 kg etanol/ton droë PS) in vergelyking met nuutpulp, was die fermentasie residu’s meer geskik vir anaerobiese vertering, wat 50% van die totale produk energie (9 288 MJ/kg ton PS) bygedra het. Buitendien, anaerobiese vertering van gefermenteerde steier het die ekstra voordeel van ʼn kort (5 tot 10 dae) biogas produksie periode. Ten slotte, sekwensiële biochemiese prosessering van papierslyk soos vergelyk met individuele prosesse, was beter om beide bio-energie en waterherwinning te maksimeer. Alternatiewelik het die sekwensiële proses die COD van die herwinde water aansienlik vererger. Gevolglik is die water wat herwin is nie onmiddellik bruikbaar sonder verdere afvalwaterbehandeling nie. Die sekwensiële benadering het ook die hoeveelheid vastestofafval beduidend verminder, wat belowende toepassings vir die landbou- en industriële sektore inhou.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/105992
This item appears in the following collections: