Novel phosphorus removal using lanthanum precipitation

Summary

V&V Walsh Meat Processors and Exporters operate a facility in Bunbury, WA, that disposes of treated effluent via irrigation to land. V&V Walsh have taken the initiative to explore chemical dosing with a novel coagulant to reduce effluent phosphorus concentrations so as to be compliant with the allowable loading limits in the DWER Licence for their irrigation areas.

This project aimed to explore the application of a novel rare earth coagulant lanthanum to meat processing wastewater and to evaluate its benefits and challenges compared to the more well-known precipitants (specifically ferric chloride). This involved an initial assessment of the current irrigation system at V&V Walsh, a review of lanthanum ecotoxicity, onsite dosing trials and sludge settling tests.

It is important to make the distinction between the use of lanthanum chloride as a coagulant in this project, and the use of Phoslock, a lanthanum-containing commercially available product. Phoslock is a clay-based material, and studies have shown that it requires doses that are orders of magnitude higher than lanthanum chloride to achieve the same degree of phosphorus removal. The results in this report are not applicable to Phoslock.

An assessment of the wastewater treatment and effluent irrigation system at the V&V Walsh facility identified phosphorus values that can be considered sustainable for two operating options. V&V Walsh indicated that they have a preference for treating the entire wastewater stream, which would require a phosphorus concentration in the irrigated effluent of 10 mgP/L.

A review of scientific and technical literature has shown that lanthanum chloride has similar toxicological impacts on the environment as common coagulants, except for freshwater aquatic systems where the free lanthanum ion appears more impacting. This environmental risk is mitigated by both the properties of lanthanum itself and by applying good irrigation practice.

Onsite dosing trials revealed that both lanthanum chloride and ferric chloride were capable of efficiently precipitating phosphorus from pond-treated effluent, with fairly low dose rates. Both are capable of achieving low phosphorus concentrations (<2 mg l) if required. to achieve a given phosphorus concentration, more ferric chloride (≈32% by mass) was required than lanthanum. however, the commercially available lanthanum chloride costs $7,000 kl compared to approximately $1,500 kl for ferric chloride, resulting in an estimated annual chemical expenditure of $190,000 for lanthanum chloride, or $60,000 for ferric chloride to reduce the phosphorus concentration from 24 mg l to 10 mg l at the site.>

Two stage dosing trials were also performed but they did not show any great benefit relative to single stage dosing. When the added operational complexity associated with two stage dosing is considered, it is clear that it is not worthwhile.

Ferric chloride had a more substantial impact on the pH and alkalinity of the effluent, meaning that pH correction may be required to protect downstream treatment processes or meet discharge limits. Lanthanum chloride did not have these effects and would most probably not require pH correction. Both coagulants produced similar quantities of sludge on a mass basis.

Sludge settling trials showed that the sludge produced by lanthanum chloride dosing settled much faster than ferric chloride sludge. This is a critical factor in the design of a chemical phosphorus precipitation system as it determines that size of settler required, which is often the most expensive capital component of such a system. It is estimated that for a settler would need to be approximately 2.5 times the size to achieve the same solids removal for ferric as lanthanum dosing. This adds substantial capital cost to the installation required for ferric chloride. Correspondingly, coagulant selection for processors becomes a tradeoff between capital and operating expenditure.

The key finding of this project is that for phosphorus precipitation and separation from wastewater, lanthanum chloride is the superior coagulant to ferric chloride on a number of criteria. However, the price premium (operating expenditure) is so substantial that few processors are likely to be able to justify using it. In addition to the price premium, the majority of chemical suppliers do not stock lanthanum chloride, potentially making it difficult for processors to obtain.

Despite this, literature suggests that rare earth elements such as cerium and lanthanum are becoming more sought after for a variety of industrial uses. This is prompting the opening of an increasing number of rare earth mines, which produce lanthanum chloride as a byproduct (USEPA 2012). If the price of lanthanum chloride were to drop substantially over the coming years, it may well find a place amongst common coagulants as a tool for processors to treat phosphorus.

In terms of future research, there is merit in investigating the effectiveness of lanthanum chloride dosing for phosphorus removal in meat processing streams that are rich with suspended solids and organic matter to determine if it is more effective than other coagulants.