Understanding and optimisation of hydrolysis and water re-use in a sugar platform biorefinery based on MSW pulp

Vaurs, Leo-Paul (2018) Understanding and optimisation of hydrolysis and water re-use in a sugar platform biorefinery based on MSW pulp. University of Southampton, Doctoral Thesis, 418pp.

Record type: Thesis (Doctoral)

Abstract

Municipal solid waste (MSW) is a widely available large-volume source of lignocellulosic material containing a waste paper/cardboard mixture which can be converted into fermentable sugars via cellulolytic enzyme hydrolysis in a waste-based biorefinery.

The composition and amenability to enzymatic hydrolysis of MSW pulps from different sources was found to depend both on the origin of the MSW, and on the conditions used in wet thermal pre-treatment (autoclaving plus washing and extraction). Glucan conversions for UK pulp were lower than for US pulp. UK newspaper was selected as a representative constituent of MSW pulp to allow assessment of the factors limiting hydrolysis and the effectiveness of pre-treatment options.

Newspaper contains a relatively high proportion (~17-25%TS) of acid insoluble material (AIM), consisting of true-lignin and pseudo-lignin, and this component appeared to contribute to the poor conversion. True-lignin is believed to affect hydrolysis by reducing the access of cellulase to cellulose and inhibiting the enzymes. Addition of acid insoluble additives adversely affected hydrolysis, indicating that pseudo-lignin could be composed of additives used in the paper recycling industry, such as wet strength resins. A strong correlation between pseudo-lignin and water retention value supported this hypothesis.

Among the pre-treatments tested (which included alkali and autoclaving), soaking in dilute phosphoric or sulphuric acid was the most effective at removing AIM and improving hydrolysis while also increasing the overall profitability of the sugar production process. Addition of surfactant (Tween, polyethylene glycol (PEG) and sodium dodecyl sulphate (SDS)) to both untreated newspaper and acid soaked newspaper greatly enhanced the glucan conversions leading to approximately 18 % extra sugar compared to without surfactant.

Modelling tools were developed to allow identification of the optimum conditions for enzymatic hydrolysis of acid soaked newspaper: these were found to be 14 % TS, 2 % enzyme dosage on a TS basis and 96h reaction time.

Alkaline pre-treatment of PHS after first hydrolysis was attempted, but was found to lead to significant material loss and low conversion, possibly due to weakened fibres. Addition of 1 % PEG in the reaction mix greatly improved the glucan conversion. The best compromise for conditions in the second hydrolysis was 10 %TS and 2 % enzyme, with duration having little effect. If the aim is to maximise profitability rather than the quantity of sugars produced, a 1-step enzymatic hydrolysis for 96h at 14 %TS and 2 % enzymes on TS should be chosen. If, however, the target is to obtain the maximum amount of sugars while minimising operational costs, a 2-step enzymatic hydrolysis of 48h each is preferred.

The potential for continuously re-using process waters after treatment in an upflow anaerobic sludge blanket (UASB) reactor was assessed, and the effect of this on enzymatic hydrolysis. After 16.5 recycling cycles glucan conversion decreased significantly, and UASB degranulation was observed. Methane yield and chemical oxygen demand removal were also negatively affected.

Data obtained from these experiments were fitted into a water-pinch model for minimisation of the fresh water requirements of a sugar platform biorefinery based on MSW pulp.