18-Env-B6 Agricultural Waste Management · December 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams, December 2016 — 04-Env-B6, Agricultural Waste Management (3 hours, open book, all questions to be attempted, 100 marks total).
Reference texts: Rynk et al., On-Farm Composting Handbook (NRAES-54); OMAFRA, Nutrient Management Act, 2002 and O. Reg. 267/03 / Nutrient Management Protocol (NMAN); Metcalf & Eddy, Wastewater Engineering (anaerobic digestion chapter); ASABE Standards (manure storage, land application equipment).
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Pumping liquid manure from a lower storage up to a higher one leaves the full static head of the upper storage available to drive backflow back down the pipeline the moment the pump stops, unless it is deliberately prevented. Several methods, each with its own residual risk, are used:
1. Check valve in the pipeline. A spring-loaded or swing check valve closes automatically when flow reverses. Risk: manure solids/debris can lodge the valve open (fail-open), and check valves require periodic inspection/maintenance to remain reliable — an undetected failure gives no warning before a spill occurs.
2. Discharge point above the receiving storage's maximum liquid level (air gap / gravity discharge). If the pipe outlet physically terminates above the highest possible liquid surface in the upper storage, gravity cannot drive flow back down the pipe even if a valve fails. Risk: the exposed above-grade discharge pipe is subject to freezing and physical damage, and may need heat tracing/insulation in winter.
3. Automatic shutoff valve interlocked with the pump. A motorized or solenoid valve that is normally closed and opens only while the pump runs. Risk: depends entirely on a fail-safe (normally-closed, power-off-closes) valve design and a reliable interlock — a fail-open valve or a control-power failure at the wrong moment defeats the protection.
4. Vacuum/backflow breaker at the discharge. Admits air to break the siphon the moment flow reverses, preventing a full backflow column from forming. Risk: the breaker itself can freeze shut or become fouled, and it protects against siphoning specifically, not against a check-valve-style forward head still being able to push liquid back through an open line.
Best practice combines at least two independent methods (e.g., a check valve AND an air-gap discharge) so that a single-point failure does not directly cause a spill.
