Application of Carbon Dioxide Stripping for Struvite Crystallization-I: Development of a Carbon Dioxide Stripper Model to Predict C[O.Sub.2] Removal and pH Changes

Article excerpt

Abstract: This research investigated the feasibility of stripping C[O.sub.2] from the digester supernatant to raise the pH, thereby reducing the caustic chemical usage. In this study, a cascade C[O.sub.2] stripper was first designed and tested, with three different synthetic solutions in a struvite recovery, crystal reactor: (1) tap water saturated with C[O.sub.2], (2) NaHC[O.sub.3] solution saturated with C[O.sub.2], and (3) NaHC[O.sub.3] + N[H.sub.4]Cl solution saturated with C[O.sub.2]. It was found that the removal efficiency of the C[O.sub.2] stripper was dependant on several parameters, such as the characteristics of the influent, including total alkalinity, temperature, and initial concentration of dissolved C[O.sub.2] gas, influent flow rate, effluent recycle rate, aeration rate, and baffle numbers in the stripper. Based on the performance of the stripper on the three synthetic solutions, a C[O.sub.2] stripping model was developed using these parameters. This model was subsequently tested in a pilot-scale facility, to predict the amount of C[O.sub.2] removal possible.

Key words: carbon dioxide stripping, cascade stripper, caustic, nutrient removal, pH, phosphorus recovery, sludge treatment, struvite, wastewater.

Resume : Cette recherche examinait la faisabilite de retirer le C[O.sub.2] du surnageant du digesteur afin d'augmenter le pH, reduisant ainsi l'utilisation de substances caustiques. Dans la presente etude, un dispositif d'elimination du C[O.sub.2] en cascade a tout d'abord ete concu et mis a l'epreuve en utilisant trois solutions synthetiques differentes dans un reacteur de recuperation des cristaux de struvite : (1) de l'eau du robinet saturee de C[O.sub.2], (2) une solution de NaHC[O.sub.3] saturee de C[O.sub.2], (3) une solution de NaHC[O.sub.3] + N[H.sub.4]Cl saturee de C[O.sub.2]. Il s'est avere que l'efficacite du dispositif a eliminer le C[O.sub.2] dependait de plusieurs parametres, tels que les caracteristiques de l'influent, dont l'alcalinite totale, la temperature et la concentration initiale de C[O.sub.2] dissous, le debit de l'influent, le taux de recyclage de l'effluent, le taux d'aeration et le nombre de chicanes dans le dispositif. Un modele d'elimination du C[O.sub.2] a ete developpe en utilisant ces parametres; il se basait sur le rendement du dispositif par rapport aux trois solutions synthetiques. Ce modele a ete ensuite mis a l'epreuve dans une installation a l'echelle pilote afin de predire la quantite de C[O.sub.2] pouvant etre retiree.

Mots-cles : elimination du dioxyde de carbone, dispositif d'elimination en cascade, caustique, elimination des nutriments, pH, recuperation du phosphore, traitement des boues, struvite, eaux usees.

[Traduit par la Redaction]

Introduction

The removal, and subsequent recovery, of phosphorus as struvite (MgN[H.sub.4]P[O.sub.4]*6[H.sub.2]O) is an innovative approach for the treatment of municipal wastewaters, especially in treatment plants employing biological nutrient removal, followed by anaerobic digestion. Struvite deposits in digester piping and pumps have been known to cause reduced hydraulic capacity and damage to equipment, leading to expensive remedial measures and process shutdowns. Although a nuisance when it forms at unwanted locations, the occurrence of struvite can be restricted through controlled struvite crystallization in a side-stream, fluidized bed reactor (Adnan et al. 2003; Britton et al. 2005).

Following successful development and operation of a pilotscale, struvite crystallizer at the University of British Columbia (UBC), the research group is now studying different aspects of phosphorus recovery, related to cost savings. The objective of this study was to first design a cascade C[O.sub.2] stripper that could be used to increase the pH of the water being treated, and then to evaluate the performance of the stripper on synthetic solutions. Finally, based on the experimental results obtained, a C[O. …