Treatment of agricultural run-off using innovative CDI filtration techniques

Treatment of agricultural run-off using innovative
CDI filtration techniques

Anthropogenic activities such as industrial discharge and agricultural run-off can negatively impact surface water quality. Agricultural run-off contaminants, other than soil particles and suspended solids are mainly nitrogen-based, phosphorus, sourced primarily from fertilisers and pesticides. Currently available treatment methods include biological treatments, aeration and filtration, however, these methods are restricted by their removal capacity, land requirement and cost. Very little research has been done on the application of capacitive deionisation (CDI) coupled with biochar in agricultural settings. This project is dedicated to investigate the capacity of this combination for removal of nitrate as a commonly existing contaminants in agriculture runoff. The application of CDI to multi-media filter layers of biochar (BC) is a promising technology to improve the nutrient adsorption capability of the BC and thus remove nutrients from the water media. The ability to easily rejuvenate the CDI-BC layers, allows for incorporation into simple backflush cycles in line with current industrial practice, whilst the increased capacity allows for a reduced number of such cycles.

This study tested the natural abilities of in-house prepared BCs sourced from agricultural waste biomass source i.e. Macadamia or as it is more traditionally known Bauple nutshells, in batch and column experiments targeting nitrate removal. The macadamia biochar (MBC) samples were pyrolysed at 900°C and 1000°C respectively, then characterised using standard techniques such as functional group identification using Fourier Transform Infrared Spectroscopy (FTIR) and physical structures analysis with a Scanning Electron Microscopy (SEM). Batch experiments found that 1000°C pyrolysed MBC achieved better nitrate removal around double than those of MBC pyrolyzed at 900°C. Column test with upward flow removed more nitrate compared to downward flow, largely due to their longer contact time. Three concentrations of 5, 10 and 15 mg/L and 3 flow rates of 2, 4 and 10 ml/min were tested applying factorial design. The lowest flow rate of 2 ml/min with the highest concentration at 15 mg/L were found to be the most effective settings for nitrate removal. A laboratory scale in-house designed CDI-MBC unit was used to assess the relative improvement to contaminant removal capacity of natural MBC.

The results of these laboratory scale tests can be used to aid the future design of a pilot-scale unit, suitable for handling typical agricultural nutrient and pesticide contamination on farm. The CDI assisted MBC tests found that with the addition of CDI, the filter can remove around three times the natural MBC capability. Incorporating CDI also prolonged effective useability of the filter. CDI-MBC reached filter saturation after 72 hours usage, compared to 5.5 hrs usage of natural MBC. Nitrate desorption was carried out after the column was saturated by being back flushed with deionised and/or tap water, using the so called ‘degaussing method’. The name is derived from the approach used to remove magnetism from test equipment by applying a strong alternating voltage. In the case of CDI we applied a square waveform of frequency 100 Hz, amplitude 0.5 V and current 0.06 A during backflush to desorb nitrate. These experiments found that the ‘degaussing’ technique recovered around 80% of nitrate in 30 minutes, while 48% and 35% of nitrate was recovered after one hour for the backflush with deionised water and tap water respectively. The CDI- MBC regenerated filter was tested for three cycles. It was found that the fresh column was exhausted after 72 hours and the regenerated filters for cycle one and two were exhausted after 60 and 48 hours, respectively.