Dear Abdul-Kadri,

Indeed the are. 

Trickling filters are conventional aerobic biological wastewater treatment units, such as active sludge systems or rotating biological contactors. The advantage of all these systems is that they are compact (i.e. applicable in densely populated urban settings) and that they efficiently reduce organic matter (JENSSEN et al. 2004). However, they are high-tech and generally require skilled staff for construction as well as for operation. Trickling filters are a secondary treatment after a primary setting process (see also septic tanksor pre treatment). 

The trickling filter consists of a cylindrical tank and is filled with a high specific surface areamaterial, such as rocks, gravel, shredded PVC bottles, or special pre-formed plastic filter media. A high specific surface provides a large area for biofilm formation. Organisms that grow in the thin biofilm over the surface of the media oxidize the organic load in thewastewater to carbon dioxide and water, while generating new biomass. This happens mainly in the outer part of the slime layer, which is generally of 0.1 to 0.2 mm thickness (U.S.EPA 2000).

The incoming pre-treated wastewater is ‘trickled’ over the filter, e.g., with the use of a rotating sprinkler. In this way, the filter media goes through cycles of being dosed and exposed to air. However, oxygen is depleted within the biomass and the inner layers may be anoxic oranaerobic.

The word filter is somehow misleading, as physical straining of solids is only marginal. The removal of organic substances occurs by use of bacterial action (UNEP & MURDOCH UNIVERSITY 2004). Therefore trickling filters are also called bio-, or biological filters to emphasise that thefiltration. Fixed film biological treatment are also used in other common treatment processes such rotating biological contactors of fixed film activated sludge systems.

For more on this topic, please see the publication contained in the following link:

http://www.sswm.info/content/trickling-filter

Hoping this will be helpful,

Rafik

Dear Abdul-Kadri,

Trickling filter has both advantages and disadvantages

Advantages

- Can be operated at a range of organic and hydraulic loading rates

- Efficient nitrification (ammonium oxidation)

- Small land area required compared to constructed wetlands

Disadvantages/limitations

- High capital costs

- Requires expert design and construction, particularly, the dosing system

- Requires operation and maintenance by skilled personnel

- Requires a constant source of electricity and constant wastewater flow

- Flies and odours are often problematic

- Risk of clogging, depending on pre- and primary treatment

- Not all parts and materials may be locally available

Please refer to the following link

http://akvopedia.org/wiki/Trickling_Filter

Regarding activated sludge process, it also has advantages and disadvantages

Advantages: Efficient removal of BOD, COD and nutrients when designed and professionally operated according to local requirements. The process itself has flexibility and numerous modifications can be tailored to meet specific requirements (e.g. for nitrogen removal). Activated sludge is the best documented and most widely used form of secondary wastewater treatment.

Disadvantages: Expensive in terms of both capital and O&M costs, requires a constant energy supply, needs trained operators who can monitor the system and react to changes immediately, and the availability of spare parts and chemicals may be an obstacle. The track record of activated sludge plants in the developing world is very poor, and few operate as designed or intended.

Plase refer to the following link.

http://water.worldbank.org/shw-resource-guide/infrastructure/menu-technical-options/activated-sludge

With my best regards

Prof. Bachir ACHOUR

I strongly agree with Prof Bachir Achour and Rafik Karaman. 

Dear Abdul-Kadri,

Biological trickling filters and the activated sludge process have demonstrated 100 years of consistent and reliable operation. Filters require an efficient influent screenings system and normally follow a primary settling stage. They can tolerate 25% over-load conditions and can be expected to produce a well nitrified effluent with suspended solids 30 mg/L and BOD 20 mg/L. They do not remove nutrients. Distribution nozzles require regular cleaning and distribution seals an annual check. The most common problems are ponding on the media surface and possibly nuisance from midges. They require 10x more land than activated sludge but use 10x less power as the oxygen is supplied by natural ventilation. Activated sludge plants typically achieve an effluent with suspended solids 15mg/L and BOD 10 mg/L and can be designed to remove nutrients. They are less tolerant of organic over-loads, toxins and in particular low dissolved oxygen all of which can result in bulking and solids washout. They require more electrical and mechanical maintenance and need trained and knowledgeable operators.

I  also agree with above answers by respected researchers.

Both of them are the conventional ones which have been in use, however reliability and successful operation varies from country to country and institutional scenario of the region.

. I have been  Technical Director for a lot of time of  WWTP,  both,  with  activated sludges  tanks and trikling filter , and I will try to  summarize my experience  in the following few lines:

The trikling filters represents a very good technology , specially for plants having the following features:

1.    N removal not required;

2.    A. E  not more than 30-40 thousands

The  vantages are :

1.    Very simple to managing ( no problems for sludge bulking)

2.    very low energy consumptions  ( if any.   A WWTP  : Imhoff- Trikling filter - Imhoff , could  operate also without any energy consumptions ) .

I have never had problems  for bed smell; but I have had problems with media clogging.  You are obliged to have a primary sedimentation and a good screening  system. But the more important clogging problems  derived from too high  organic load : more than 1.0 kg of BOD5 for cubic meter of media per day. However,  you can have clogging problems only  with   traditional media (stones) but not if you use modern plastic  media. It is possible to  achieve also good nitrification, but you  need to reduce  the organic load  from 0.25-0.30 kg/mc  of BOD5 per  day ( that is enough    for BOD5 reduction)  until   less than  0.1 ( The given results are valid for traditional media).  Moreover,  if you achieve nitrification, you do not have any  available process for nitrates removal ( if it is necessary)  , i f you have available  only trikling filters. In comparison  with  RBR  a trikling filters  WWTP is much more   reliable and resilient

I have used trikling filters as first biological treatment    ( high load: more than 1.0 kg/mc per day of BOD5)  followed by activated sludge process  for N removal;  with  very good results; both for industrial ( pharmaceutical, food factory, etc.)  and civil  sewage treatment .

A may paper  about this  experience  is included in my web space (www.studiopezza.it) but, sorry, it is written only in  Italian language !     

The key to answering this question is to match the process to the influent stream to be treated. This means considering both size (PE) and composition. Trickling filters re best suited to relatively small streams with low solids content. They don't respond at all well to industrial wastes and are easily poisoned. Activated sludge can be scaled massively as required. With pre-treatment (primary settlement to remove gross solids) they are reasonably robust.. Activated sludge is relatively inefficient for phosphorus removal but this can be solved with ferric dosing. They can also be used in combined aerobic/anoxic  zones to obtain biological phosphorus removal. Not all activated sludge plants operate in the same way. Many operate as simple continuous reactors, but a growing alternative is to run them as a sequencing batch reactor. See http://www.unitedutilities.com/liverpoolwwtw.aspx for example. Note also that hybrid reactors e.g. http://www.watermaxim.co.uk/biological-aerated-filter.php such as BAFs are essentially fixed film like the trickling bed but suspended like the activated sludge. All in all there is no perfect answer  it realyy requires matching the process to the stream to be treated.

Apart from trickling filters and activated sludge methods of secondary waste water treatment, oxidation ponds or the so called artificial wetlands or lagoons can also be employed as secondary treatment mechanisms

Some additional recommendations for the use of filters triklings I suggest are:

To avoid the disadvantages of the use of trikling filters is to make a team of this type combined with placed anaerobic systems at the bottom of the filters trikings with recycle between both the trikling filters dissolve and oxidize gases from the lower anaerobic system, also they favor the presence of facultative bacteria and in turn allow biological sludge which will be processed at the time that the organic load decreases or is a company that stops working during weekends accumulate.

The cost of pumping and bindings pumping systems are reduced considerably using known pumping system using air lift tubes immersed in water tanks supplied with air at the bottom. Which enhances oxidation of water.

The use of ozonizers for feeding air into the pumping or elevating levels substantially improves the process by eliminating oxidation compounds from the anaerobic system, reducing odor and making a small oxidation of the most sensitive substances oxidation and substantially increasing the concentration of oxygen in the feed water filters triklings. He will think of the presence of peroxides, but reality has broken down and only the presence of dissolved oxygen.

The ozonation system is very convenient to use it for final disinfection to obtain an acceptable sanitary water discharge and avoid using hypochlorite, which are able to produce organochlorines in the residual.

It is convenient to start thinking about the use of absorption wells in well-treated wastewater to reintegrate groundwater levels being folding down by water extraction pro supply wells and take advantage of the filtering that the same terrain makes the water. And in some communities take advantage of tax opportunities practices have increased availability of underground water through wells

Construction costs of the systems are folded using structures of steel tanks, treated and with suitable paints, which allows modifications and also has a recovery cost from the sale of scrap, not the use of concrete or plastic. The economic package of triklings filters for its weight and support area of the attached biomass is using packaging waste PET sectioned, also lightweight materials and high contact area that is not compacted by the weight of packaging biomass and loading of process.

If there is the presence of active substances that interfere with different biological processes is desirable before the wastewater entering the treatment plant deactivate wastewater and the most practical methods for my experienced is by the use of activated carbon or natural zeolite, or in extreme cases a pre treatment with ozone or H2O2  si hay disponible and ultraviolet light.

In engineering practice it commands normally some sort of natural selection. So normally the processes of treatment more used are surely the more reliable at the date. The area of wastewater treatment has been observing a very rapid development in the last decades, but sometimes it may occur some delay between the practice and the achievements obtained by scientific research. Many developments have been obtained recently with activated sludge processes, particularly with respect to tertiary treatment that go much beyond the conventional secondary treatment processes, so it looks that currently activated sludge is still prevailing.

I concur with David. The quantity, the composition of the wastewater and the goals to be reached with the treatment are essential when deciding the processes to be used.

Dear Abdul-Kadri,

The straight answer to your "most" question is NO. It is not because some trickling filters TF and conventional activated sludge CAS technologies are being used over 50 years that would make these most reliable at least not in terms of treatment performance, process stability and ease of operation and maintenance. Based on my 40 years worldwide expertise, I would rank the black-box TF much lower than CAS unless you like musquitoes and don't care too much about performance, operation and maintenance especially on how to recover from a accidental oil breakthrough or toxic shock. A well designed CAS would rank much higher but still below the more advanced activated sludge AAS developped and applied over the last 20 years and I am not referring to MBR or SBR which are basically CAS implemented with a membrane to replace a clarifier (MBR) or using 1 large tank for sequential biotreatment and clarification over time (SBR). True AAS provide much higher performance by also capturing and biodegrading recalcitrant compounds "slow food" (such as color: dyes, lignin, ...) in addition to readily biodegradable soluble organics "fast food" removed by all biological systems. In addition, true AAS are more stable than CAS by integrating cyclic regeneration combined with continuous metabolic and kinetic selection of a highly active and diverse micro-population in dense biosludge (SVI < 50 ml/gDS).

Regards, Bruno

The above observation can be shown to be incorrect about the trickling filter if using an appropriate design for trickling-filter solids contact (TF/SC) with a suitable irrigation and flushing design .  The wastes solids are much easier to manage and SVIs lower than CAS.  Appropriate design with recycle can provide for nutrient removal of ammonium and phosphorus.  The latter has over 15 years demonstrated experience in a TF/SC with recycle plant for domestic wastewater.

Over the last 40 years all TF's in the EU that I knew have been replaced by CAS or AAS because of its inherent limitations even when well designed and built. A TF is a biofilm system which implies that it can only metabolize "fast food" and mineral nutrients (ammonia, phosphate, ...) at sufficient concentration (concentration gradient needed) to diffuse through the biofilm as to get to the encapsulated microbes (mainly bacteria). In addition TF is a blackbox for process control because it is impossible to monitor and control the operating conditions at microbial level within the biofilm as opposed to suspended growth systems like CAS and AAS. Hence the only process control in TF is the recycle flow rate.

 I agree about the waste solids in terms of SVI as compared to most CAS systems because CAS has no/little means to control the biosludge quality in terms of density and microbial activity as opposed to AAS. That's why many CAS system struggle with light and bulking sludge (SVI > 200 ml/gDS).