Sequenced anaerobic - aerobic treatment of hemp pulping wastewaters

Abstract

Biological treatment is an indispensable instrument for water management of non-wood pulp mills, either as internal measure to enable progressive closure of water cycles, or as end of pipe treatment. In this thesis, the sequenced anaerobic-aerobic treatment of hemp ( Cannabis sativa L. ) pulping wastewaters is described, with a focus on the treatability of thermomechanical pulping (TMP) effluents and soda pulping black liquors. The research was performed within the framework of the Dutch Hemp Programme, which aimed to develop environmentally safe and economically feasible pulping processes as a measure to explore non-food markets for arable farming.Pulp and paper industry wastewaters are highly heterogeneous, depending on the feedstock and the pulping processes utilized. Hemp TMP wastewaters were found to be non-toxic to methanogens and highly suitable for sequenced anaerobic-aerobic treatment, which enabled 63-66% COD removal at loading rates up to 27 g COD/ lUASBd. Hemp black liquors on the other hand, were somewhat more problematic, due to the high methanogenic toxicity, causing 50% inhibition of acetoclastic methanogenic activity (50% IC) at concentrations ranging from 2-6 g COD/ l , which is 10 to 100 times lower than the black liquor concentrations in industry and comparable to the methanogenic inhibition of wood pulping wastewaters.Apolar hemp extractives (resinous compounds) were observed to be the main source of inhibitory substances in hemp black liquors. Lignin derivatives on the other hand, were less important for methanogenic inhibition, since the main share of lignin in hemp black liquors was non-toxic. Despite the high methanogenic toxicity of hemp black liquors, anaerobic treatment was feasible, provided that adequate dilution was applied. Anaerobic treatment of diluted hemp stem wood black liquor (HSWBL) facilitated recovery of methane and high treatment efficiencies at high organic loading rates. Subsequent aerobic post-treatment provided almost complete removal of BOD, however COD-removal was limited due to the presence of recalcitrant lignin. Additionally, substantial detoxification was obtained after anaerobic-aerobic treatment. The major removal of inhibitory compounds was accomplished in the aerobic step.The effective reduction of methanogenic toxicity during sequenced anaerobic-aerobic treatment of hemp black liquors was used as a detoxification strategy denominated upfront dilution, recirculating the aerobic effluent to dilute the incoming influent to sub-toxic concentrations. The feasibility of the upfront dilution method was demonstrated in an experiment, in which direct anaerobic treatment and sequenced anaerobic-aerobic treatment with upfront dilution were compared. Direct anaerobic treatment of 12 g COD/ l HSWBL led to almost complete inhibition of the methanogenic activity within 14 days. While recirculation of 75% of the aerobic post-treatment effluent for upfront dilution of the toxic HSWBL, enabled anaerobic treatment at loading rates up to 21.5 g COD/ lUASBd without noticeable inhibition of methanogenesis. Extensive detoxification was confirmed during anaerobic-aerobic treatment of 20 g COD/ l HSWBL recirculating 86% of the aerobic effluent. COD and BOD removal was 47% and 68%, respectively, after anaerobic treatment; and 74% and 97%, respectively, after anaerobic-aerobic treatment at an overall loading rate of 3.6 g COD/ l d, while 30-35% of the incoming COD was recovered as methane.Lignin removal during anaerobic-aerobic treatment of hemp pulping wastewaters was remarkably high and ranged up to 44%, of which globally half was obtained anaerobically. Studies on hemp TMP wastewater revealed that lignin removal during anaerobic treatment corresponded to the lowest molecular weight derivatives, indicating that biodegradation was the major removal mechanism. Subsequently, aerobic post-treatment of hemp pulping wastewaters caused extensive polymerization of lignin to molecular weights in excess of 34 kD, which was associated with strong increases in wastewater color. Autoxidative polymerization of polyphenols formed out of lignin by anaerobic bioconversion is proposed as a mechanism for the observed increase in lignin molecular weight and color. Apart from the highly stable high molecular weight lignin-derived humic COD, biologically treated pulping effluents are relatively clean, which offers benefits for lignin removal during tertiary treatment, such as reduced fouling of ultrafiltration membranes and improved precipitation with divalent salts.Comparison of anaerobic-aerobic treatment and aerobic treatment shows that treatment efficiencies and detoxification were similar. The anaerobic-aerobic treatment system however, provided 50% lower surplus sludge production, production of methane as an energy source (0.16 m 3/kg COD removed ), less nutrient dosage and substantial reductions in aeration costs.The results of this research demonstrate that sequenced anaerobic-aerobic treatment is a suitable technology for the treatment of hemp pulping wastewaters. Upfront dilution effectively reduced inhibition of methanogenesis by extractive compounds during anaerobic treatment, whereas the observed increase in lignin molecular weight after biological treatment offers benefits for lignin removal during optional tertiary treatment.</p