Slurry Storage Calculator

How the slurry storage calculator works

This tool builds an indicative planning estimate of how much slurry and dirty water may need to be stored over a winter-style assessment window (by default built from October through March). It brings together livestock slurry production, parlour washings, rainfall on yards and uncovered stores, and roof water routed to the slurry system, then compares those volumes with the working capacity you enter for existing and proposed stores.

Monthly slurry volume ≈ livestock slurry + parlour washings + dirty yard runoff + store rainfall + roof water.

Livestock slurry m³ ≈ head × m³/week per head × storage weeks  ·  Rainfall contribution m³ ≈ area m² × rainfall mm ÷ 1000

Existing vs revised slurry storage planning

On many farms the useful question is not only “how much storage do I need?” but “what improves my storage position?” Comparing an existing layout with a revised plan helps you test roofing dirty yards, diverting clean roof water, covering stores, reducing wash water or enlarging capacity—before committing to capital works.

What counts towards slurry storage volume?

• Livestock excreta collected as slurry
• Parlour and dairy washings that enter the slurry system
• Runoff from uncleaned yard surfaces draining to store
• Rainfall landing on uncovered store surfaces
• Roof water (and similar) routed into slurry storage
• Other dirty water sources you deliberately include

Slurry store types and capacity

The calculator supports earth bank lagoons (frustum-style volume with a fixed freeboard allowance in the method), rectangular concrete stores, circular / tower tanks, and a custom known capacity when you already have a certified or surveyed working volume. Freeboard is the safety ullage kept empty at the top of a store; it is not available working storage—always follow structural and regulatory guidance for your store type.

Rainfall and dirty water

Rainfall can add a very material volume where yards and stores are uncovered or where clean water is allowed to mix with dirty areas. Use a month-by-month rainfall profile that reflects your location; the defaults here are only a planning placeholder, not a meteorological guarantee.

Parlour washings

Wash water can dominate storage on some dairies, especially high-pressure systems. Use farm meter readings or realistic estimates per cow milked per day across the housing period where you can.

NVZ and slurry storage rules

Nitrate Vulnerable Zone rules, SSAFO (England), and devolved Scottish, Welsh and Northern Irish requirements can specify minimum storage periods, spreading constraints and record-keeping. Requirements differ by nation, farm type, and whether land is in an NVZ. This page does not encode country-specific legal tests; treat results as a planning guide only and confirm with current official guidance.

Livestock manure nitrogen loading

Where livestock types include indicative kg N/year per head, the tool can estimate total manure N and, if you enter a farmable area, an indicative kg N/ha/year figure. An indicative 170 kg N/ha/year livestock manure loading threshold is sometimes referenced in NVZ discussions—the real assessment depends on spreading areas, organic manures, exports, and detailed rules not replicated here.

Ways to reduce slurry storage pressure

• Roof dirty yards and divert clean water away from the slurry system
• Separate clean and dirty drainage routes
• Cover stores to exclude rainfall
• Reduce unnecessary wash water and fix leaks
• Increase engineered storage capacity where appropriate
• Review mechanical separation performance where used
• Check that drainage outfalls and routes match the farm map in practice

Always comply with environmental permitting, field-spreading rules and professional advice—this list describes common engineering measures, not permission to discharge illegally.