A hired pilot plant lets you test ammonia stripping on your own wastewater before committing to a full-scale installation. In a proposal for a containerised unit treating about 1 US gallon a minute (roughly 5 m³/day), the minimum hire was 90 days, with a process engineer, and the output was a firm design and performance specification for the full-scale plant.

The challenge: proving a process on a real waste

Digestate from farms and food waste varies widely. Ammonia strength, solids content and scaling metals all differ from site to site. A full-scale stripper designed on assumptions risks fouling or underperforming. A pilot trial replaces the assumptions with measurements.

In this case the aim was to strip ammonia from the liquid pressed out of agricultural digestate and recover it as aqueous ammonia, in order to develop a reference site and supply trial quantities to local customers. The pilot would run alongside an ultrafiltration (UF) pilot, which removes solids from the digestate before it reaches the stripper.

The approach: a containerised pilot plant

The design basis

• Flow: about 1 US gallon per minute.
• Ammonia nitrogen: 1,000 to 5,000 mg/l in the feed.
• Feed temperature: about 35 to 40°C, pH 7.5 to 8.5.
• Solids: about 100 mg/l after UF, with 500 mg/l as the preferred maximum for the pilot.
• Target: an effluent below 200 mg/l ammonia nitrogen, and an ammonia product above 8% nitrogen.

What is in the container

The plant is a scaled-down version of the full process, packaged in a standard ISO container:

• An inlet buffer vessel, feed pump and filter.
• An electrically powered feed heater, because a pilot rarely has a waste heat source. A full-scale plant could use biogas instead.
• A heat-recovery exchanger between the incoming and outgoing liquid.
• The stripping column with packing, insulation and cleaning systems.
• An air blower and a small steam boiler.
• A cold-water scrubber with its own chiller, where the ammonia is absorbed to form aqueous ammonia.
• A PLC, control panel and instruments.

The container is insulated for operation in a cold winter climate. The pilot needs a three-phase electrical supply of about 75 kW, water and a feed and discharge connection.

Why a trial is structured in four tasks

1. Planning and site evaluation. Agree objectives, success thresholds, measurements, staffing and the programme of works.
2. Delivery, installation and commissioning. Ship the unit, connect the utilities, balance the stripper with the UF pilot and commission with a commissioning engineer in attendance.
3. Operation and data collection. Run campaigns, assess the data for validity and accuracy, and report regularly.
4. Final assessment. Consolidate the data and produce a firm design, operating method and performance specification for the full-scale plant, with a cost assessment.

The agreed deliverables followed a timetable. A programme of works came in week 1, and a method statement and operations manual in week 4. Shipment, arrival and installation followed, with commissioning from about week 25.

What the pilot tells you that a design cannot

• Fouling rate. How quickly the packing scales, and whether a cleaning cycle is needed. The proposal expected no cleaning in the first three months, unless it was done to check the packing condition.
• Real removal and recovery. Whether the effluent and product meet specification on the actual waste.
• Interaction with upstream treatment. How the stripper behaves when solids carry over from the UF stage.
• Operating demands. What a technician has to do and how often.

Practicalities

The hire is an operated trial, so an engineer runs it. After the minimum 90 days it can be extended month by month, for up to a year, with 21 days’ notice so that an operator can be arranged. It can be moved to other farms during the trial under the engineer’s supervision.

Several items were excluded and need to be planned by the customer: transport from the port, offloading, the electrical and water connections, ammonia product storage, sample analysis, staff for routine operation, permits and local taxes.

What to consider for your own project

1. Define success before you start. Agree the effluent and product targets, and how they will be measured.
2. Make sure the pilot feed is representative. Take it from the actual process, not a bench sample.
3. Prepare the site. Power, water, drainage and a laboratory should be ready before the unit arrives.
4. Allow for shipping time. In this example, the unit took roughly 20 weeks to reach the port.
5. Plan for storage of the product. Even small volumes of ammonia solution need suitable containers.

For the full-scale design that followed this type of trial, see our post on recovering ammonia from digestate.

FAQ

How long should a pilot trial run?

At least 90 days in this proposal, which gives time to see how the packing fouls and to gather data across a range of feed conditions.

Do I own the pilot plant?

No. It is a hire, and the unit returns to the supplier at the end of the trial.

Can I use the product during the trial?

Yes. A stated purpose was to produce trial quantities of aqueous ammonia for potential customers.

Who operates the pilot?

The supplier provides an operator for the trials, working with online monitoring and regular site visits. The customer provides staff for routine site tasks and sample analysis.

Thinking of a pilot trial?

Tell us about your waste stream and we will outline a trial programme.