Biotransformation of phosphogypsum in wastewaters from the dairy industry

Abstract

The biotransformation of phosphogypsum by stationary cultures of sulphate reducing bacteria (SRB) in dairy wastewaters (sterile and non-sterile) enriched in phosphogypsum, was studied SRB were isolated from soil contaminated with petroleum-derived products and from wastewaters from the petroleum-refining industry, taking into account that these environments are characteristic for the bacteria of the studied group. The biotransformation products formed were investigated using biological and mineralogical methods. Sulphides in the cultures were determined using the iodometric method, sulphates with the hot barium method, COD by the dichromate method. Determinations involving post-culture sediments and fluids, Ca, S, Sr and P were made using the following analytical procedures: IPC emission spectrometry with induced excitation in the medium and X-ray. Analysis of post-culture sediments was made using a DRON-2 X-ray diffractometer.

Eleven communities of microorganisms with varying degrees of effectiveness were obtained, biodegrading 58% to 98% of the organic contaminants in non-sterile wastewater, with simultaneous biotransformation of about 2.5 g phosphogypsum/L.

Introduction

Phosphogypsum is an industrial waste product formed from apatites or phosphorites in the production of phosphoric acid. The main component of phosphogypsum is gypsum (CaSO₄ × 2H₂O), accompanied by bassanite (CaSO₄ × 0.5H₂O). About 2.5% are various additional contaminants, unreacted parts of ore, residual sulphuric and phosphoric acid and other components (Varjo et al., 2003). Phosphogypsum, which contains about 50% sulphates, can undergo biotransformation in cultures of sulphate reducing bacteria (Azabou et al., 2005, Wolicka and Kowalski, 2006b).

The biotransformation of phosphogypsum by sulphate reducing bacteria (SRB) is effective when a liquid medium containing various organic compounds being a carbon source for bacteria, is used. Various liquid organic wastewaters can be used as such a medium, provided they meet certain conditions: contain organic compounds that are susceptible to biodegradation and have a low sulphate content.

These requirements are met by dairy wastewaters. Their main organic contaminant are carbohydrates, mainly lactose (about 31% dry wt.), protein (24%, including casein) and fats (42%). The composition and volume of dairy wastewaters depends primarily on the kind of products made, measures taken to minimize the amount of waste, water economy and cleansing agents used. The pH of dairy wastewaters depends on the nature of the end product and can range from 6.6 to 12.2 (Alvarez-Manteos et al., 2000, Carrasco et al., 2004).

The presence of SRB was determined in reactors purifying dairy wastewaters (Baena et al., 2000, Hernandez-Eugenio et al., 2000) but their role is rather small in view of the low concentration of sulphates that averages 220 mg/m³ (in Section 2, Media). The addition of sulphates to dairy wastewaters, in our case of phosphogypsum, causes them to become high sulphur-containing, as in the case of wastewaters from the production of molasses (2.9 g SO₄/L), citric acid from cane sugar (2.5–4.5 g SO₄/L) or the wood industry (1–2 g SO₄/L) (Colleran et al., 1995). High concentrations of sulphates in wastewaters preclude their purification by the methanogenesis method. For this reason attempts are being increasingly often made to purify such wastewaters by employing sulfidogenesis (Azabou et al., 2007a, Azabou et al., 2007b, Wolicka and Kowalski, 2006a, Wolicka and Kowalski, 2006b).

The simultaneous biodegradation of two industrial wastes: phosphogypsum and dairy wastewaters as the liquid phase to dissolve phosphogypsum seems an interesting solution from the economical point of view. The costs involved when simultaneously biodegrading two industrial wastes being a hazard to the environment are always lower than for each of them separately.

In view of the above, the aim of the present study was to isolate, anaerobic sulfidogenic communities of microorganisms from various environments and to determine the possibility of the biotransformation of phosphogypsum in cultures of sulphate reducing bacteria with simultaneous purification of dairy wastewaters.