Nutrient Irrigation and Management Plan Case Study

Nutrient and Irrigation Management Plan (NIMP)


Irrigation is one of the most efficient and sustainable methods for the disposal of treated wastewater within the premise. In Western Australia, the permit to irrigate treated wastewater is given by the Department of Environment Regulation (DER). To acquire the necessary permit, a nutrient and irrigation management plan (NIMP) is required to be submitted and approved by the DER. The purpose of an NIMP is to provide justification of the irrigation regime to ensure that minimal to no environmental impact will occur to the site’s surrounding.  

The Problem

Currently most environmental consulting firm uses the recommended nutrient loading mentioned in the Water Quality Protection Note (WQPN) #22 as the basis of the NIMP. This approach is taken to minimise the time consumed to prepare the NIMP and it is the current industry standard. However, the result of this approach often leads to unnecessary large irrigation area requirement which is often not available at the site and limits future opportunities for the applicants.

EEI’s Approach

Upon reviewing the WQPN #22 document, we understand that the recommended nutrient loading rate is applicable to be used if there is insufficient information regarding the wastewater quality and quantity and the site. Given that sufficient information is available, modelling of the movement of irrigated water and the fate of the constituents can become the basis of the NIMP. EEI took this approach and have developed a specialist model which can calculate the minimum irrigation area required based on the irrigation scheme and characteristic of the underlying soil. If irrigation area is limited or there is a target irrigation area, the model can calculate the required treated wastewater quality which is suitable for the target irrigation area. EEI has applied this model to develop our NIMP and to review other firms’ NIMP. The NIMP developed using this model and the review of other firms’ NIMP has been accepted by the DER.

Case Studies

Vernon Arms Tavern, Baldivis WA

Vernon Arms Tavern has an onsite wastewater treatment system to treat the wastewater produced from the brewery. The water is treated to the conditions as required by the DER to be classified as a Class A+ water. To dispose the treated wastewater, Vernon Arms Tavern has been instructed by the DER to develop an NIMP. Initially, they engage another consulting firm to develop the NIMP. However, the recommendation for the NIMP suggests that an area of 8,000 m2 is required. The implication of the NIMP means that the land that has been planned for future development is no longer available as it will be required by the irrigation. The Tavern engaged EEI to explores the options that can reduce the required irrigation area. Through our model, we suggested that by reducing the phosphorus concentration from the wastewater, the irrigation area required can be reduced to 2,000 m2. The revised NIMP was submitted to DER and accepted. The result means that the development of the tavern can go as planned.