Once wastewater has been treated, it needs to be discharged somewhere - it doesn't just disappear or evaporate.
The choices are: discharge to land, freshwater bodies (lakes, rivers or streams), estuaries, and the ocean. The soils, terrain and adjacent water bodies determine the most feasible choice.
Currently, Central Hawke's Bay's wastewater is discharged to our rivers. However, our early conversations with community reference groups have told us that this is no longer acceptable and that to meet the cultural and environmental aspirations of the community, we must work toward land discharge.
Land discharge systems provide the ability for water to be spread over an area to infiltrate into the soil. As the water passes through the soil water can be retained and taken up by plants, nutrients can be filtered out and stored in the soils for plants to use, and pathogens can be filtered out. The effectiveness of the system is dependent on:
- the volume and how much is applied,
- the type of soil and whether it is a gravely riverbed or loamy soil,
- how wet the soil is, and
- what is growing on the site.
Below, you can read about the considerations that have been taken when developing the options for our wastewater discharge:
- Nutrients & contaminants: What are the key nutrients and contaminants currently of concern and require limiting during discharge - is the waterway able to accept any further contaminants?
- Concentration level: What are the target values and acceptable concentrations to be achieved downstream after reasonable mixing?
- Cultural: What is the impact on cultural values?
- Public use: What are the impacts on users - (who are the users, what are their uses and relevant values, when do they use the waterway, and what changes from the existing state would the discharge cause)?
- Public awareness: Is Public Health protection needed (eg spread of disease from contact with contaminated water or consumption of contaminated fish), and what is the relevance of recreational values?
- Outfall pipe: What is the method of entry for the discharge (e.g.via an open drain or a submerged pipe within the river bed)?
- Location: What is the location of entry for the discharge and its sensitivity for causing concerns or risk of damage from erosion?
- River flow: What is the flow rate of the waterway compared with the discharge – what dilution rate is likely? Are there tidal cycles, flood events, and seasonal low flows and high flows which affect how much dilution is possible and what times of day and year are best suited to accept the discharge.
- Seasonal effects: Is the flow rate and quality of treated wastewater being discharged affected by storm events and seasons?
- Discharge timing: What is the timing of discharges – will discharge events be restricted to times of day such as falling tides, above median flows, and/or hours of darkness?
- Storage: Is storage for deferred discharges possible – How much storage volume will be needed? Does the existing WWTP have enough freeboard or what size pond will be needed for additional storage? Where will an additional pond be located?
- Location: How close is the land to the WWTP and how many sensitive neighbours are nearby?
- Terrain and elevation: Gentle slopes or flat land at similar height to WWTP are ideal.
- Soil properties: Water holding capacity, drainage rate, and particle size or soil type will determine loadings rates.
- Land use: Will wastewater irrigation integrate with the current land use (eg reserve, farm, or forestry) or will the land use need to change to accommodate the wastewater irrigation? How will harvesting or stock grazing regimes integrate with irrigation regimes?
- Land area: How much land is available after subtracting buffers and unsuitable land?
- Climate: What are the normal and extreme ranges of rainfall, soil moisture deficit, and wind for each month and year? How will these variations limit irrigation design and operation?
- Method of discharge: Examples are subsurface dripper lines, fixed irrigators, travelling irrigators, border dyke or infiltration basin, and (artificial) wetland basin.
- Application rate and purpose: Choices are deficit for zero drainage while maximising plant growth, non-deficit for some drainage and loss of nutrients to groundwater, rapid discharge for disposal to groundwater or wetland, and land passage prior to entering waterways.
- Timing of discharges: What seasons and weather or soil moisture conditions will restrict or prevent discharges (eg wet soils, heavy rain, or strong winds)?
- Storage for deferred discharges: How much storage volume will be needed? Where will an additional pond be located?
- Emergency or large storm contingencies: What will happen to excess wastewater volumes and what limits would be appropriate?