2.5 How is the treatment performance of sub-surface planted gravel filters?

The performance of planted gravel filters depends on a range of crucial factors. These factors include the general design, flow pattern, hydraulic and organic loads, pre-treatment, sizing, retention time, temperature, quality of the constructed components, plant species used and quality of operation and maintenance.

To ensure the proper functioning of the filter, which is intended as a secondary or tertiary treatment step, a pre-treatment or primary treatment step is required to settle sludge and reduce organic matter (COD and BOD). Suitable pre-treatment options may include a septic tank, biogas settler, anaerobic baffled reactor, and anaerobic or trickling filter.

Regarding the filter itself, the flow pattern of the filter system is an essential aspect that impacts the treatment efficiencies for different contaminants. Vertical planted gravel filters generally reach higher BOD, COD and TSS removal efficiencies due to better oxygen penetration in the filter media and longer contact time due to the narrower and taller design. In comparison, horizontal planted gravel filters have proven more efficient in removing nitrogen and phosphorus. Pathogen removal will depend highly on the retention time for both filter types. The longer the retention time, the higher the pathogen reduction rate.

The following table lists approximate results that HPGF and VPGF can achieve.

The treatment performance of both types of planted gravel filters is temperature-dependent; an increase or decrease in the temperatures leads to improved treatment effectiveness. BOD degradation rate generally increases or decreases by about 10% per °C.

Additionally, the treatment performance is influenced by organic and hydraulic loading, and it tends to improve when these loads are reduced by increasing the overall retention time. A VPGF should be fed in batches, dividing the overall inflow into several portions. The optimal hydraulic load for VPGF is typically recommended to fall within the range of 2.5 to 5cm per batch. Lower hydraulic loads may pose the risk of inadequate distribution of inflowing wastewater, while higher hydraulic loads could decrease treatment performance.

In the aspect of de-/nitrification, VPGFs are recognised for their capability to provide oxygen, thus facilitating the transformation of ammonia into nitrate through a process known as nitrification. With optimal oxygen supply, these systems can achieve remarkable nitrification rates, reaching levels of up to 90%. However, denitrification tends to be less efficient. In such cases, nitrogen remains as nitrate in the effluent, and the overall nitrogen removal rate typically caps at around 30%. To enhance nitrogen removal, especially if it’s a critical treatment objective, it’s advisable to consider pairing a vertical planted gravel filter with a horizontal planted gravel filter. Additionally, the option of flow recirculation can be explored.