Treating thermomechanical pulping wastewater with biomass-based fly ash: Modeling and experimental studies

The possible application of fly ash (FA) to ameliorate acid mine water (AMD) for irrigation of potato (Solanum tuberosum L.)

Some areas in Johannesburg abounds with mine wastes namely, acid mine drainage (AMD) as well as fly ash (FA), which are by-products of gold mining and coal burning, respectively. Studies show that a solution formed through mixing these wastes neutralises the acidity of AMD and is an alternative source of irrigation. While studies show improved growth and yield of plants irrigated with fly ash-amended AMD, there are rarely sufficient studies conducted in South Africa showing evidence of altered pH of AMD and that food crops irrigated with fly ash-amended AMD exhibit improved concentration of essential nutrient elements. In this study, AMD was sourced from a gold mine in Johannesburg and fly ash collected from a coal-burning power station in the Mpumalanga Province, mixed at 1:0, 1:1, and 3:1 (w/v) of fly ash to AMD and used to irrigate potatoes. The objective was to assess whether the solutions of FA-amended AMD alter the pH of the AMD and to evaluate if irrigating potatoes with the aforementioned improve the concentration of essential nutrient elements and heavy metals in the tubers. Results show that the pH of AMD was increased in the 1:0 and 1:1 solutions but decreased in the 3:1 solution. The concentrations of Pb and Co were decreased in tubers irrigated with the 50 % AMD and 75 % AMD while that of Ni and Cd were markedly increased in tubers irrigated with solutions of fly ash-amended AMD. In the main, the potato tubers exhibited significantly higher concentrations of Al, Mo, Cu, Ca, Mg, and Zn when irrigated with fly-ash-amended AMD. The pH range levels from FA-AMD treated samples were within the acceptable pH range (5.5-6.5) which is acceptable for water that could be used for irrigation of crops. Also, the decreased Co and Pb and improved concentration of essential nutrient elements indicate that the constituents absorbed large quantities of the heavy metals while releasing the nutrients. In conclusion, the selected fly ash has proven as an alternative low-cost readily-available, affordable, and accessible adsorbent that neutralize the acidity of AMD, decrease the concentration of heavy metals, and increase the concentration of essential nutrient elements. Importantly, the liming potential among other traits of the fly ash improved the quality of the AMD such that the wastes were proven in this study suitable to irrigate potatoes.

Granulated biomass fly ash coupled with fenton process for pulp and paper wastewater treatment

The work describes the combination of granulated biomass fly ash (G_BFA) with Fenton process to enhance the removal of adsorbable organic halides (AOX) from pulp bleaching wastewater. At optimal operating conditions, wastewater's chemical and biochemical oxygen demand (COD and BOD₅, respectively) and colour were also quantified, and operating cost of treatment assessed. For the first time, raw pulp bleaching wastewater was used to granulate BFA, instead of water, reducing the water footprint of the treatment. Five wastewater treatment setups were studied: (i) conventional Fenton process; (ii) G_BFA application; (iii) simultaneous application of G_BFA and Fenton process; (iv) sequential treatment by G_BFA followed by Fenton process; (v) sequential treatment by Fenton process followed by G_BFA. The latter yielded the highest AOX removal (60-70%), whilst COD was also reduced (≈15%) and wastewater biodegradability (BOD₅/COD) was enhanced from 0.075 to a maximum of 0.134. Another positive feature of the proposed solution was that G_BFA were successfully recovered and reused without regeneration, yielding similar AOX removal compared with fresh G_BFA. The operating cost of removing 1 g of AOX from the pulp bleaching wastewater by the optimal treatment setup (60-70% removal of AOX) was 14-26% lower than the operating cost of conducting Fenton process alone (50% removal of AOX).

Improvements on activated sludge settling and flocculation using biomass-based fly ash as activator

Biomass-based fly ash and wastewater are undesired products of the pulping industry. Recently, the use of biomass-based fly ash as an adsorbent (i.e., a valued material) for constituents of wastewater effluents was reported. In this work, the settling performance and properties of activated sludge were studied in the presence of fly ash. Upon mixing, fly ash increased the zeta potential of the sludge from -31 mV to -28 mV, which was due to the release of cationic ions from fly ash in the sludge suspension. The sludge settling and its flocculation affinity were improved through the complexation of flocs and released cation ions from fly ash. The relationships between the protein/polysaccharide (PN/PS) ratio and the content of extracellular polymeric substances (EPS) as well as the ratio and the properties of the sludge flocs were determined. A correlation between the total loosely bound-EPS (LB-EPS) content and the effluent suspended solids (ESS) (Pearson's coefficient, r_p = 0.83) was observed. The performance of sludge flocculation and settling were much more closely correlated with LB-EPS than with tightly bound EPS (TB-EPS). Scanning electron microscopy (SEM) analysis of sludge flocs before and after EPS extraction showed that the sludge flocs contained a large number of microorganisms, mainly Bacillus and Cocci. The amount of LB-EPS had an adverse influence on bioflocculation, effluent clarification and sludge settling affinity. The sludge properties had a moderate relationship with the PN/PS ratio of LB-EPS. Also, no correlation could be established between the ratio and the TB-EPS content.

Isolation of lignocelluloses from the spent liquor of thermomechanical pulping process with fly ash and cationic polymer

In this work, fly ash (FA) and polydiallyldimethylammonium chloride (PDADMAC) were utilized to treat the spent liquor (SL) of thermomechanical pulping (TMP) process in an effort to remove its lignocelluloses. The incorporation of PDADMAC into the system reduced the dosage of FA required for achieving acceptable lignocellulose removals. The maximum lignocellulose removals of 81%, 78%, 56%, 53% and 97% were achieved for lignin, hemicellulose, COD, BOD, and turbidity via treating SL with 100 g/L of FA at 25 °C for 60 min and subsequently treating with 100 mg/L of PDADMAC at 25 °C for 30 min, respectively. Comparing the two-step processes, FA pretreatment with PDADMAC post treatment was more effective than the two step process of PDADMAC pretreatment and FA post treatment. In this case, the FA pretreatment generated metal-organic complexes, and the addition of PDADMAC facilitated the formation of large flocs that could be separated from the system readily. A one stage process of combined PDADMAC and FA was less effective than the two-stage process of FA and PDADMAC treatments in removing lignocelluloses from SL.

Enhancement in biological treatment of pulping wastewater by fly ash

Sequential batch reactor (SBR) is a simple and flexible activated sludge process for industrial wastewater treatment. Also, biomass-based fly ash is a fairly plentiful and low-cost waste available in the pulp and paper industry. For treating wastewater of the pulping industry through a more efficient and economic approach, the integration of fly ash in the SBR process was investigated in this work. In this study, fly ash dosages of 0.2 wt% and 0.6 wt% were maintained in SBR1 and SBR2 systems, respectively, for treating the wastewater of the pulping industry. The findings indicated that adding fly ash to the bioreactors improved the settling and flocculation affinity of activated sludge without having any significant effect on the performance and the stability of the biological process. Lignin and color removals were 90.9% and 95% in SBR1 and 92.9% and 97.5% in SBR2, while the removals in the control reactor were 85.3% and 91.5%, respectively. The alkaline ions, such as Ca²⁺ and Mg²⁺, leached out from fly ash, which improved the sludge's properties, and this leaching reduced 22.1% and 40.5% of alkali consumptions in SBR1 and SBR2, respectively. The structure of sludge flocs in the reactors with and without fly ash was also studied. This technology is environmental friendly, cost-effective and suitable for a full-scale implementation in existing aerobic biological processes.