Alkaline Solubilization of Coconut Shells for Use in the Denitrification Process

Abstract

Abstract

This research investigated the suitability of alkali-treated coconut shells as a carbon source for denitrification. Suitability was assessed through an exploration of specific denitrification (k_N) and carbon consumption (k_C) rates, in addition to kinetic constants. The rates and/or constants were calculated from batch tests conducted in five denitrification flasks. Each flask contained three components, with the first being activated sludge that had been washed to remove residual organic matter and that provided a denitrifying biomass. The second component was the electron donor, which consisted of neutralized supernatant from carbon-extraction flasks. These flasks had coconut shell fragments (2 mm ≤ x ≤10 mm) which had been soaked for 7 days in NaOH (pH = 12); yielding a net release of approximately 700 mg L⁻¹ chemical oxygen demand (COD). The final component was the electron acceptor; namely, 55 mg L⁻¹ of nitrate (NO₃⁻-N). The addition of this quantity of nitrate yielded a nonlimiting carbon-to-nitrogen (C:N) ratio of 2.6. Denitrification was successful and occurred in two distinct phases. The first was an “acclimation phase” having mean k_N and k_C values of 0.060 ± 0.015 mg NO₃⁻-N mg VSS⁻¹ day⁻¹ and 0.587 ± 0.069 mg COD mg VSS⁻¹ day⁻¹, respectively. The second phase denitrified three times as fast with a k_N value of 0.168 ± 0.063 mg NO₃⁻-N mg VSS⁻¹ day⁻¹. The k_C value, however, was only 0.044 ± 0.128 mg COD mg VSS⁻¹ day⁻¹, suggesting either autotrophic denitrification and/or a dynamic balance between carbon released via cell lyses and carbon consumed by heterotrophic activity. The rate data was evaluated against zero- and first-order kinetic models; however, only the second phase of the nitrate-removal data could be fitted, and that was to zero-order kinetics (mean k₀ value of 11.2 ± 1.8 mg L⁻¹ h⁻¹).