Trickling Filter Plant
Category / type: fixed bed, secondary treatment
Application: small civil settlements, accommodation facilities.
Population Equivalents (PE) served: from 2 up to 50 (Packaged and/or site assembled) and more than 50 tailor-made.
Normative references and guarantees: EN 12566-3, d.o.p. According to Regulation (UE) n.305/2011.;
Material: concrete, polyethylene, polypropylene, glass-reinforced plastic (GRP).
A trickling filter is a fixed bed biological reactor with one or more filtration chambers.
Wastewater percolates through the plastic filling medium inserted into the filter. Organic matter is decomposed by bacterial biofilm growing on the filtering mass.
Percolating filters are widely use as secondary treatment for civil or assimilable wastewater, downstream of mechanical pre-treatment or settling and can be used as tertiary treatment for industrial wastewater to remove organic pollution.
It’s used downstream of pre-treatment as screening, sedimentation, grease trap, septic tank or Imhoff tank.
The purification process is based on separation by gravity sedimentation and biodegradation by biological actions.
How It Works
Trickling filters are both aerobic and anaerobic depending on technical solutions.
Leachate is used with a low concentration of suspended solids concentration, therefore a sedimentation phase is needed upstream to the filters.
In case of high BOD concentration, biogas may be produced by anaerobic bacteria and it must be vented.
BOD reduction is from 50% to 80%, even up to 90% in some cases. Nitrogen removal is limited and normally does not exceed 15% in terms of total nitrogen (TN). Even pathogen reduction is not significant.
Hydraulic retention time (HRT) should be from 1.5 up to 2 days for the pre-treated black water.
Depending on purification target, research shows that it would be sized from 0.5 mc / PT to 1 mc / PT, volume composed by filling material and water.
Anaerobic trickling filter construction is similar to septic tank and there may be more chambers to increase process efficiency.
Most common filter materials are gravel, stone chopping, brick or pumice stone. High specific surface plastic material is the most widely used solution for best performance.
Percolating through filtering mass, suspended solids come in contact with bacterial biomass grown on filling material that decompose the organic matter with small retention times. The greater the surface, the quicker the digestion takes place.
Specifics shape of plastic filling materials provide from 90 to 300 m2 per m3 occupied.
There are downward flow and upward flow filters. In the first case cleaning is better, in the second case there is less risk of losing bacteria.
Hydraulics retention time (HRT) is the main sizing parameter.
Internal pipes must be designed to limit turbulence; flow distribution must be homogeneous and the oxygen requirement must be guaranteed for aerobic reaction.
A secondary settling is needed for dead biofilms dragged by flow.
The main parameters for civil wastewater plants (or equivalent) project is population equivalent (PE); normally it refers to 60 g BOD5 per day and a flow rate from 150 to 250 litre per day.
Population equivalent (PE or sometimes used p.e.) is a conversion value which aims at evaluating non-domestic pollution in reference to domestic pollution fixed by EEC directive (Council Directive 91/271/EEC concerning Urban Waste Water Treatment) at 60 g/day related to BOD5.
European legislation defines Population total (PT) as a sum of population and population equivalent (PE).
A proper wastewater characterisation is possible for incoming organic loads and nutrients better estimation, considering the possibility of dosing additives for biological reactions.
Operation and maintenance
A trickling filter requires a 6-9 months start up periods to achieve a full efficiency treatment, because of slow bacterial growth (especially anaerobic).
Inoculation with similar bacteria is possible to reduce start up time. Sludge may be taken from plants or septic tanks. The operating flow rate is achieved gradually.
Hazardous waste or chemicals must be treated very carefully: they can block the biological reactions and require a new start up procedure.
Foams and sludge level should be monitored periodically. Bacterial films, due to a fast growth or to a usual death, could create an excess of sludge that can block the filter.
The system must be periodically maintained by authorised company.
Anaerobic process produce biogas, that, if not stored, is needed to be vent also to avoid odour problems.
considering the possibility of dosing additives for biological reactions.
The European standard EN 12566-3 “specifies requirements, test methods, the marking and factory production control for packaged and/or site assembled domestic wastewater treatment plants (including guest houses and businesses) used for populations up to 50 inhabitants…”
The manufacturer shall state the nominal organic daily load expressed in kg of BOD5 or BOD7 per day and the nominal hydraulic daily flow expressed in cubic meters per day.
Plant’s Nominal Designation is the proper daily organic load value (QN) (3.7).
The small wastewater treatment plant must demonstrate compliance with the performance of efficiency … and to operational data declared by the manufacturer: the declaration must be expressed in percentage.
Note also that the structural resistance and the hydraulic efficiency must also be tested only on a representative model of the product line. Structural resistance is tested on the bigger, supposedly as the one with the worst characteristic, hydraulic efficiency is tested on the smaller, presuming that represents worst efficiency.
The standard describes statics and hydraulics methods to determinate resistance and hydraulic efficiency.
Information to be included in the test reports and in the CE mark are also detailed (ZA.3).
In short, for small domestic waste water treatment plants up to 50 PT (therefore for activated sludge plants), it is required Declaration of Performance (DoP), as provided by the EU Regulation. 305/2011.
Features and benefits
The Trickling filter plant is a very flexible solution, that can be used for small and medium civil settlements or for single or double family houses.
Constant flow improved efficiency, as for all biological wastewater treatment and activated sludge process.
The trickling filter can treat grey and black domestic wastewater or equivalent. Pre-treatment as septic tank, grease separator or Imhoff is needed.
It’s a secondary treatment but it can be also applied as post treatment or as further treatment after activate sludge section.
Trickling filter require less areas than natural solution as phytoremediation and low or no energy consumption.
Desludging needs vacuum truck and it must get close to the tanks for cleaning operations.
System is made in a watertight tank, material depends on groundwater level and soil characteristic.
Plant can be installed underground or aboveground.
Trickling filters can be built in almost all environments, eventought special adaptations for cold climates are required due to lost of efficiency. An underground installation, a proper insulation, a reduced effluent recirculation and an improved distribution technique can lessen the impact of cold temperatures.
Nutrients and pathogens reduction is quite low and to increase effluent quality further treatment is needed.
- good yield to shocks and hydraulic load;
- low or no power supply;
- low operating costs;
- long useful life;
- good BOD and suspended solids reduction;
- low excess sludge production;
- stabilized sludge;
- small footprint (underground).
- low nutrients and pathogens reduction;
- it may require tertiary treatment;
- clogging risks (if the pre-treatments are insufficient);
- more complex cleaning operations than septic tank;
- low population density zone;
- long start up times.