3.4 What is the expected treatment performance?
The treatment performance of a trickling filter is impacted by the type and characteristics of the filter media, organic and hydraulic loading rate, air supply (aeration), temperature, filter depth and general operating conditions.
Concerning the filter media, a specific surface area (SSA) and porosity of the media play a key role in the adequate performance of the system. The SSA quantifies the extent of surface area accessible for biofilm development and is expressed in m2 of filter media available per m3 of the filter volume. At the same time, porosity defines the available void spaces through which air and wastewater can flow expressed in %. Increased specific surface area and porosity will allow for higher loading rates, taller filters, and lower area requirements. Different characteristics for filter media are characterised in the table below.
The trickling filter’s treatment capacity and type are defined by the organic and hydraulic loading rates. Trickling filters can be classified into low-rate and high-rate filters. While effluent recirculation is not always provided in low-rate trickling filters, it is common for high-rate trickling filters to improve efficiency, mainly for diluting the influent.
The differences between types of trickling filters and their treatment performance are summarised in the following table.
| Parameter | Standard (Low-Rate) Filter | High-Rate Filter (with stone media) | High-Rate Filter (with plastic media) |
|---|---|---|---|
| HLR (m3/m2/d) | 1 – 4 | 10 – 40 | 15 – 90 |
| OLR (kgBOD/m3/d) | 0.08 – 0.22 | 0.36 – 1.8 | 0.32 – 3.0 |
| Depth (m) | 1.5 – 3 | 1 – 2 | Up to 12 |
| Filter media | Rock, gravel with a diameter of 5 – 8 cm | Rock, gravel with a diameter of 5 – 8 cm | Plastics such as cut plastic bottles or special plastic material |
| Recirculation ratio | 0 | 1 – 3 | 4 – 12 |
| Dosing Intervals | < 5 min | 15 – 60 sec | Continuous |
| BOD Removal Efficiency (%) | 80 – 85 | 40 – 80 | 65 – 85 |
| TSS Removal Efficiency (%) | 80 – 85 | 65 – 80 | 65 – 80 |
| Nitrification | Well-nitrified | Little nitrification | Little nitrification |
High reduction rates of organic matter and suspended solids can be achieved by trickling filters. For BOD and Total Suspended Solids, up to 85% are possible.
Some nitrification can also be achieved by trickling filters at low organic loading rates and suitable temperatures. The Nitrogen is mainly oxidised in nitrate; however, the nitrification process can be initiated by lowering the organic loading rate for a BOD <20 mg/L; in this case, a total nitrogen removal of up to 35% is reported to be possible. Reduction in phosphorus is achieved by up to 15%.
Trickling filters are reported to remove 90 to 95% of the viruses, bacteria, and cholera vibrio; 0% of the leptospirosis; 50% of Entamoeba histolytica; 70 to 100% of the Ascaris; 50 to 99% of the schistosome ova; and 50 to 95% of the taenia ova.
Further Reading:
- Comprehensive Overview on DEWATS Effluent Post-Treatments: Explanation of the design, sizing, operation and maintenance of slow sand filters. P. 23 – 34 (Link)
