Ammonia scrubber design and use
There are several techniques to control ammonia depending on the type of livestock and production techniques. For housed livestock where emissions can be channelled, a reduction in the ammonia emissions can be achieved by using a scrubber.
The design of the scrubber is crucial to its effectiveness.
Ventilation rates for livestock sheds vary through the growing cycle and will also depend on the weather conditions. The capacity of scrubbers will need to be calculated based on the anticipated amount of air flow needed to maintain a healthy environment in the shed throughout the crop. Factors like thinning and length of growing period will need to be taken into account, along with the design of the ventilation for the shed to ensure that the abatement equipment is sized correctly.
For example, ventilation for laying hens is more consistent through the cycle, but will still vary depending on weather conditions. The design of the abatement will need to consider whether the livestock is housed inside or is free range, but the ventilation will need to be managed to direct airflow through the abatement equipment at all times.
Optimum ventilation will maintain a healthy growing environment by controlling the temperature in the shed and limit levels of dust particles (the law sets this at 1mg per square metre), carbon dioxide (3,000 parts per million), ammonia (20 parts per million) and humidity (70-84% depending on external temperature). If you are growing for a particular market, eg RSPCA Assured, you will need to meet the requirements of that scheme.
Depending on the market you are providing for, the peak total weight of the crop will vary. Once you have established the peak weight, you can begin to calculate the appropriate ventilation rate. The ventilation rate for your type of livestock needs to be taken into account.
Ventilation rate calculation example
For broilers, the amount of air you need to remove per hour is calculated as follows:
(Bird capacity of shed multiplied by average peak live weight in kg) multiplied by 4.5 multiplied by 0.7 = amount of air that the system needs to be capable of treating per hour in metres cubed.
4.5 cubic metres per hour per kilogram live weight is the ventilation rate recommended in the EU for broilers. The maximum calculated airflow is only needed for the small proportion of the growing cycle when the birds are approaching maximum weight and when the weather is warm. We accept that the overall required amount of ammonia reduction can be achieved by scrubbing 70% of the maximum calculated air flow and so we multiply the maximum flow by 0.7.
For a typical 50,000 bird broiler unit where birds are grown to 2.0 kg
50,000 multiplied by 2.0 multiplied by 4.5 multiplied by 0.7 = 315,000 cubic metres per hour
If you thin a proportion of the crop, you will need to calculate the scrubber capacity based on the ventilation need at the time the crop is at its maximum weight.
For both broiler and layer housing it is accepted that emergency ventilation may be required that bypasses the scrubber during times of high external temperatures, the scrubber design should take into account normal UK seasonal variations and the temperature at the location.
Storing the scrubber liquor
You will need a tank that meets the requirements of the Silage, Slurry and Agricultural Fuel Oil Regulations to store the liquor that is drained from the scrubber. The capacity must be adequate to hold the maximum you may need to store at any time depending on whether you intend to use the liquid ammonium sulphate as a fertiliser on your own land or send it off site. You will need to add the nitrogen content to your farm’s nutrient management plan if you intend to use it on your own land and maintain records of where it is used offsite.
Installation and maintenance
The equipment must be installed and operated in accordance with the manufacturer’s instructions. Regular maintenance must be carried out to ensure that the conductivity and pH of the scrubber liquid remains within the correct parameters. Procedures for operating, monitoring and maintaining the equipment must form part of the site’s management system and anyone involved with managing the farm must receive training in how to ensure that everything is running properly.
Records must be made (and kept) of checks on the equipment so that they are available for inspection by NRW, the farm assurance scheme inspector or planning authority.
Continuous monitoring is needed of the scrubber liquid conductivity and pH, pressure drop, external air temperature, air flow and whether other ventilation is in use.
Other monitoring should include the volume, type and concentration of acid purchased and used; the volume of water being used by the system and waste liquid produced; lighting programmes in addition to all of the standard internal and external environmental data for the individual sheds. An example system records the following every minute:
- Ventilation rate (cubic metres per hour)
- Temperature from four in-house sensors
- In-house relative humidity
- Outside temperature
- Outside relative humidity
- Pressure in Pascals (Pa) each minute;
- pH of scrubbing liquor
- conductivity of scrubbing liquor
In-house ammonia monitoring could be used to show the level of ammonia present before abatement.
Your environmental permit may contain conditions that limit the mass release permitted from the unit per year. This will be calculated as a percentage reduction from a standard emission factor for the type of unit.
Your permit may also contain pre-operational conditions to ensure that the equipment that is installed meets the requirements, and that the people operating it have received suitable training. The following will be included in the permit.
- pre-operational condition to ensure that the design is agreed before the operator orders any equipment.
- pre-operational condition to ensure that NRW is informed of when the unit will be stocked.
- pre-operational condition to ensure that operatives have been trained
- monitoring condition to included process monitoring requirements
- reporting condition to require results of process monitoring to be reported to NRW quarterly.
- evaluate whether current emissions monitoring requirements are feasible for the various types of abatement equipment.
The following limits will be taken as evidence of compliance and could form part of your permit in future.
- conductivity: less than 150 microsends per centimetre (µs/cm)
- pH: less than 3.5
- appropriate scrubbing liquor removal rates