Tannery waste usually contains a high content of chromium, since this metal is the active ingredient for tanning leather. In many areas, the wastewater from this industry is led to the municipal sewage works following some form of pretreatment. In Italy, the Consorzio Di Disinquinamento Di Solofra, CODISO, has chosen another treatment option. Moreover, due to new legislation, CODISO is looking for a new solution for treating sludge.

The CODISO case

A group of more than 60 tanneries concentrated at Solofra, in the south Italian province of Avellino, discharge their wastewater into a treatment plant located centrally. This plant is owned by CODISO.

The wastewater treatment plant consists of the following stages:

* Homogenisation tank

* Pre-settling tank. The sludge collected here will be included in the organic sludge stream

* Chemical treatment, using a mixed aluminium-iron salt and anionic polymer

* Primary settler. Chemical sludge is collected here

* Biological treatment (activated sludge), with activated carbon

* Secondary settler. The excess sludge joins the organic sludge stream

The effluent is discharged into the Sarno River. The basin of this highly polluted river has attracted special attention from the national and local authorities.

The total annual amount of sludge, both chemical and organic, produced at Solofra wastewater plant is 35,000 tonnes per year, corresponding to 26% DS (9100 tonnes DS/year). The chemical sludge part makes up 12,000 tonnes and contains most of the metals, while the remaining 23,000 tonnes is organic sludge, consisting basically of organic matter.

At present, all this sludge is disposed of as landfill at rather a high cost – L4550 billion (€2.4 million). In addition, Italian legislation has become stricter and dumping sludge with a high content of organic matter will be forbidden within a short time. As a result of these facts, CODISO asked Kemira Kemwater to develop a project for sludge treatment in order to reduce the amount of sludge produced, and to improve the quality of the final sludge and recycle valuable components.

The Kemwater solution

Kemwater is presently performing a feasibility study for CODISO along the following lines:

* The organic sludge will be treated thermally in a tunnel with a retention time close to 30 days, which will result in a fertiliser with a high nitrogen content. This product is considered ‘green’ under Italian legislation and, according to the opinion of other manufacturers of this kind of product, has good market potential.

* The chemical sludge will be the subject of an Eco Process that will yield three products:

1) An aluminium-based salt, that can be used as raw material by Kemwater

2) A chromium oxide, which can be marketed to the tanneries. The chemical sludge contains 1.5-2% chromium. With this treatment, chromium is not only separated from the sludge, thereby improving its quality, but can also be recycled in a profitable way

3) An organic residue, which can probably be treated together with the organic sludge

The prospects for the CODISO Project are very promising, as there would be almost zero discharge, and the final products have a market value. The feasibility study will be completed in the first half of this year.