Trickling Filters

The last twenty years have seen substantial improvements to biological trickling filters, prompted by introduction of modern structural plastic media instead of traditional stone fill.

Plastic media has transformed the treatment capability of trickling filters, making them two to three times more effective for a given volume. Reductions of more than 95 percent are now possible in BOD and ammonia, making trickling filters a viable option in many instances.

A key advantage of trickling filters, compared to other treatment technologies, is their very low power requirements. In fact, the * Global Water Research Coalition (2011) has identified trickling filters to have the lowest energy usage of any secondary processes in the hierarchy of wastewater treatment processes.

This energy advantage, combined with operational simplicity, and resilience to manage variable loadings, has proven to be effective. Many WWTP’s in Europe, USA, NZ and elsewhere select trickling filters as their process solution of choice, in some cases even where capacity is greater than 500,000 EP.

Technology Overview

Trickling filters operate by distributing wastewater over media (plastic or traditionally stone/rock) that is typically 2-6m deep.  Bacteria and micro-organisms grow on the media and feed off the contaminants in the wastewater as it percolates through the profile, thereby providing treatment. Trickling Filters are designed to maintain aerobic conditions by maintaining “free draining” conditions within the filter, and drawing air into the system through the updraft from ventilation holes at the base of the tank. This passive aeration mechanism is the underlying reason why trickling filters are not dependent on energy intensive blowers, achieving such low energy operation.

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Although a range of different media can be used in industry, structured plastic media has revolutionised design and operation of trickling filters.  It significantly increases the surface area for microbial biofilms per unit volume, achieves void ratios >95% (e.g. 95% air) and avoids blockages deep within the profile.  These characteristics are essential to maintain efficient oxygen delivery to the microbes, and drive efficient performance.

Filter flies, a common problem with stone media or random packed media, are not an issue with structured plastic media. This is because the open matrix structure is an unsuitable habitat, so fly lava cannot proliferate.

Advantages of Trickling Filters compared to Activated Sludge

Modern trickling filter designs offer many benefits that Activated Sludge processes cannot. These include:

  • Low energy (50-70% less energy than equivalent performing Activated Sludge);
  • Less susceptible to changes in effluent quality, toxicity and shock loads: Attached growth processes (e.g. Trickling Filters) are more resilient to variable wastewater conditions than suspended growth systems (e.g. activated sludge);
  • Ability to handle hydraulically variation in peak and dry weather flows. The hydraulic retention time within the Filter is not substantially affected by flow variation as a fixed volume suspended growth tank; water “falls” by gravity from top to bottom, irrespective of the flow;
  • Simple to operate, and;
  • Low sludge production, sludge thickens and dewaters easily.

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Construction underway of the 1ML/day Narrogin WWTP Trickling Filter

History of Trickling Filters in Australia

In Australia, Trickling Filters have been used since at least the 1930’s, generally by the municipal waste industry.   Whilst trickling filter systems were common until the 1970’s, they were mainly comprised of stone media, which presented a number of short-comings. Some of the disadvantages were poor oxygenation particularly deeper down in media bed; blockages on the surface; proliferation of filter flies; odour; and inadequate treatment performance.  As WWTP’s faced increasing pressure, over the last thirty to forty years, to deliver higher performance outcomes, they opted for a range of activated sludge technologies. These superseded many old trickling filter plants when upgrades were needed.

Although trickling filters have been dismissed in recent times due to their historic poor performance, development of high performance plastic media over the last fifteen to twenty years has transformed trickling filter technology.  These improvements have addressed many of the short comings associated with stone media.  Modern plastic media Trickling Filters are two to three times more effective than stone media systems of the past, and require far lower energy than comparable activated sludge processes.

Internationally, developments in trickling filters have been recognised, and the technology has seen a resurgence in many parts of the world over recent decades.  These advances have taken longer to adopt in Australia, but with growing pressure to reduce energy costs, interest in the benefits of modern trickling filters is growing.

An example of modern plastic media Trickling Filter solutions being embraced in Australia is the upgrade of Water Corporations’ 1ML/day Narrogin WWTP in Western Australia. The project entails design and construction of a plastic media “nitrification” trickling filter.

Low Nutrient Wastewater with Trickling Filters

Biological nutrient removal requires a process train that incorporates a combination of aerobic and low oxygen conditions.  Trickling filters are an aerobic process, and are highly effective at organic removal and ammonia nitrification, but by themselves they are unable to deliver low Nitrogen or Phosphorous outcomes.  However, when trickling filters are complemented by separate anoxic processes, such as anoxic bioreactors or surface flow wetlands, they are able to achieve very low nitrogen results.  Water Corporations’ Narrogin WWTP upgrade is designed to achieve final TN<5.8mg/L by integrating the nitrifying trickling filter with a 2ha anoxic surface flow wtland.  Nitrogen levels could be lowered further if required, by modifying the trickling filter volume and wetland area.

The 3ML/day South Lismore WWTP in northern NSW is an example of using an old stone media trickling filter and a surface flow wetland for successful nitrogen removal.  The graph below shows 4 years of EPA monitoring data for total nitrogen at the end of the WWTP. The ninetieth percentile TN over a four year period was 8.0mg/L, half the ninety percent licence limit of 15mg/L.  Using plastic media in the trickling filter and implementing better design of the wetland, such as water level control, could improve the performance still further.

Trickling filters don’t provide significant removal or reduction of phosphorous, except for minor amounts that are absorbed into the biofilm.  Phosphorous reduction can be achieved easily by integrating chemical dosing such as alum or ferric into the process train.  The addition of chemical dosing doesn’t contribute significantly to overall energy consumption of the plant, so overall low energy benefits are still afforded by trickling filters.

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Ingolstadt WWTP

Alternative solutions for aerated bioreactors in Activated Sludge process trains

Another application of trickling filter technology is to integrate it into mainstream activated sludge treatment trains, as a substitute for any bioreactor that requires an aerobic process.  The advantage of substituting trickling filters for aerated suspended growth bioreactors is that it can significantly reduce energy consumption of a WWTP, by delivering biological outcomes without the need for energy intensive blowers.

This ‘hybrid’ activated sludge/trickling filter solution has been adopted at the 275,000 EP Ingolstadt WWTP in southern Germany.  Four plastic media nitrification trickling filters, each 3,400m3, have been integrated into the secondary activated sludge process train, as an alternative to a second stage aerated bioreactor. The WWTP has achieved significant energy savings as a result.