A new method to treat farm dairy effluent to produce clarified water for recycling and to reduce environmental risks from the land application of effluent

Purpose

There is increasing public concern about the amount of fresh water used on New Zealand dairy farms and the adverse impacts that farm dairy effluent (FDE) has on the environment. FDE mainly consists of fresh water (99%) with a small amount of solid material (1%). There is an opportunity to treat FDE to remove the colloidal solid material and recycle the water to wash the farm yard.

Materials and methods

A new method of treating FDE has been developed. The method uses a coagulant to flocculate and settle the colloidal particles in the FDE and thus produce the clarified water for recycling. The research and development programme involved three stages: (i) standard laboratory jar tests to establish the amount of coagulant required to treat the FDE, (ii) large tank tests to confirm the effectiveness of the coagulant at a larger scale and (iii) construction and testing of two pilot plants. A pasture field trial was conducted to test the effect of the clarified water and treated effluent on plant production.

Results and discussion

The new method of treating FDE was highly successful in removing the colloidal material and producing clarified water that can be recycled to wash the farm yard. The average turbidity of the clarified water from the first pilot plant was 52 nephelometric turbidity units (NTU) (a 97% reduction in NTU compared to the original FDE) and the average Escherichia coli concentration was 9 cfu per 100 mL (a 99.99% reduction compared to the original untreated FDE). The average total-phosphorus (P) concentration of the clarified water was 1.8 g m^?3 (94% reduction), dissolved reactive phosphorus (DRP) was <?0.1 g m^?3 (99% reduction) and total nitrogen (N) was reduced by >?70% (from 200 to 61 g m^?3). The average turbidity of the clarified water from the second pilot plant was <?20 NTU (a 99.5% reduction). There was no adverse impact from the land application of the clarified water or the treated FDE on plant growth.

Conclusions

A new method for treating farm dairy effluent was successfully developed that produced clarified water that could be recycled to wash the farm yard and had a lower potential risk of environmental impacts when applied to land than untreated FDE. Land application of the clarified water or the treated FDE had no adverse impact on plant growth.