Client – Dry Rendering Plant Operator
Dry rendering plants process animal by-product materials for the production of tallow, grease and meat and bone meal. Raw meat by-product is heated in either a continuous or batch process to dehydrate the material and release fats that are captured and later used for livestock feed and other uses. Water recovered as part of the dehydration process is typically condensed, collected and treated before discharge.
The operator of a dry rendering plant was experiencing difficulties with wastewater discharge compliance due to high levels of sulphide in the treated effluent.
A direct approach was originally tried, adding hydrogen peroxide to the final discharge stream in order to oxidize the sulphides, reducing them below the limits impose by the local water authority (1 mg/L). This methodology only provided the expected results approximately 50% the time and required a high dose of chemical, increasing overall treatment cost.
A new approach was taken using a methodical analysis of process flows with the aim to identify the process stream with the highest concentration of sulphide, and reduce this pollutant at the source. This approach allows cost-effective treatment of the smallest possible volume of water.
Analysis showed that the condensate stream from the dehydration process contained over 20 mg/L of sulphides. As the highest contributor of sulphides this was the most effective stream to address in order to reduce downstream levels.
To reduce the amount of hydrogen peroxide consumed a review of the sulphide chemistry was undertaken. The results of this review showed that:
- Sulphide exists in three forms in the hydrogen sulphide system – hydrogen sulphide (H2S), bisulphide ion (HS-), and sulphide ion (S2-). The sum of these three compounds is commonly referred to as total sulphide, with the concentration of total sulphide reported in equivalent units as mg/L S2- . Total sulphides are herein referred to as “sulphides”.
- At pH values over 9.0 the predominant reduced sulphur form is bisulphide (HS-) which is soluble in water. In contrast, at pH values lower than 6.0 the predominant species is hydrogen sulphide (H2S), the gaseous form of reduced sulphur. At a pH of 7.0, half of the dissolved sulphide species for any given concentration is bisulphide, and half remains as hydrogen sulphide, as shown in Figure 1 below.
- Hydrogen sulphide (H2S) can be removed from the medium through aeration.
The recognition that sulphide removal was possible through aeration at pH values of around 6.0 or lower allowed the development of an alternative treatment system which involves a reactor vessel with constant pH, and air injection.
After implementing the new reactor to treat the condensate stream sulphide levels were reduced by 87%. The effect of this reduction was such that at the point of discharge wastewater contained less than 10% of the allowable levels of sulphides.
The simplicity, and efficient placement of the system in the low volume, high sulphide stream, allows for low cost of operation and long term reliability. Critically, the success and effectiveness of the solution relied on a pragmatic and methodical analysis of the underlying process issues leading to the simplest possible solution.