Development of a bioreactor system using a pine bark matrix for the removal of metal ions from synthetic aqueous solutions.

Abstract

Many industries use, or produce, metal-containing solutions which must be treated for reuse or discharge to sewer. One such treatment is biological and both living and dead materials have been investigated for the abstraction of metal ions from solution. Studies on systems containing only a single biosorbent are well documented, and mostly involve optimisation of biosorption capacities and metal uptake rates through modification of Biological Support Particle (BSP) size and surface characteristics. Literature on dual biosorbent studies is sparse. The commercial application of biosorption technology in wastewater treatment remains largely unexplored and unexploited. The primary objective here was to assess the potential of forced-upflow packed-bed bioreactors, containing dual biological sorbents, for treating a synthetic wastewater containing copper, zinc and cadmium, at both laboratory- and pilotscale. Pine bark was selected as BSP since it is an abundant, relatively cheap, agricultural waste product in South Africa, and is known to sorb metal ions. Initial experiments aimed to optimise biofilm development on the pine bark surfaces, since microbial biomass is also known to sequester metal ions. Systems comprising either one, or both, these biosorbents were compared for their efficiency in metal removal. The effects of type, size, and state of decomposition, of the pine bark, the addition of supplementary nutrients (Voermolas) and the mixing conditions, on the metal biosorption capacity and reaction kinetics of the systems were also studied. All experiments were conducted at an initial metal concentration of 100mg.ℓ⁻¹with both composted and uncomposted pine bark as BSP. The former supported microbial colonisation and resisted biofilm sloughing, but degraded rapidly causing engineering difficulties. Uncomposted pine bark showed the same ability, but was also physically more robust. Organic compounds leached from the pine bark did not hinder microbial colonisation of the BSP; rather they served as additional nutrients. Literature studies suggest that these compounds would not significantly compromise the COD or increase the toxicity of the final effluent. Biofilms developed without supplementary nutrients, but Cd²⁺ and Zn²⁺ were sorbed more effectively in bioreactors containing Voermolas (39% and 38% Cd²⁺ removal, 36% and 32% Zn²⁺ removal, in 0.2% and 0.1% Voermolas solutions respectively) than in unsupplemented systems (25% Cd²⁺ removal and 20% Zn²⁺ removal). Conversely, Cu²⁺ was removed most efficiently in the absence of supplementary nutrients. Based on biosorption of the target metal ions, 0.1% (v/v) Voermolas was the most effective concentration of supplementary nutrients. Raw, un-colonised pine bark nuggets (16-24mm), and plastic bioballs (commercially available, bespoke BSP), were compared in laboratory-scale bioreactors by measuring the decrease in residual metal ion concentrations over time, and changes in the solution pH. These experiments showed that the two BSPs did not differ significantly in their performance as a support matrix, or as a metal sorbent (30.6% and 32.6% of metal ion remained in solution when using bioballs and pine bark respectively). However, the presence of a biofilm on both these BSPs, improved the overall performance of the bioreactors significantly (for the bioball BSP, residual metal ion levels decreased from 30.6%, in the absence of a biofilm, to 11.0% with a biofilm present. Similarly, for the pine bark BSP, residual metal ion levels decreased from 32.6%, in the absence of a biofilm, to 7.3% with a biofilm present). A cost comparison of the two BSPs showed that raw pine bark nuggets were available at less than 0.1% of the cost of the bioballs. At pilot-scale, modelled kinetic data compared poorly with experimentally determined results, but minimum residual metal concentrations for Cu (1.7mg.ℓ⁻¹) and Zn (4.2 mg.ℓ⁻¹) were below South African (eThekwini Municipality) regulatory limits for discharge to sewer (5mg,ℓ⁻¹ for both), and sea outfall (3mg.ℓ⁻¹ Cu and 20mg.ℓ⁻¹ Zn). However, for Cd the final residual metal concentration (5.6mg.ℓ⁻¹) was above the regulatory discharge threshold for any receiving system. Although some of the effluents from the system investigated could not be legally released into the municipal sewer system without further remediation, the study showed that a system combining living and dead biomass in a single reactor is capable of significantly reducing dissolved metal concentrations in synthetic wastewaters without temperature or pH adjustment. Furthermore, such a system can operate at pilot-scale, where a pine bark matrix represents a significant cost saving over conventional plastic BSPs.