Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | AUGUsT 2014 | 37 In the know Getting to grips with trickling filters load as it ensures excess sludges are expelled. However, it is becoming increasingly common to see large channel, cross flow media used in roughing filters. Secondary/carbonaceous treatment Trickling filters are ideal for secondary water treatment – i.e. the removal of BOD and ammonia to levels that can be discharged into watercourses without compromising the condition of the receiv- ing waters. The higher loading rates applied to secondary filters are typically between 0.2 kgBOD/ m³/d to 1 kgBOD/m³/d, depending on the required effluent quality. With appropriate topographic layout, trickling filters can operate in gravity feed mode without any additional source of power. No other aerobic water treatment process offers this advantage. Tertiary nitrification water treatment Trickling filters used in tertiary nitrification are designed to meet tighter ammonia discharge consents. The nature and type of biomass that establishes itself within tertiary nitrification filters produces negligible amounts of solids. The latest generation of trickling filters allow for different types of media to be layered accord- ing to anticipated biomass production with large channel cross- or vertical flow media (for BOD re- moval) at the base layers and small channel cross flow media, forming a secondary filter, at the top. The primary trickling filter for BOD removal would be designed to meet a BOD of <25 mg/L, while the increased surface area available in the secondary filter provides for higher nitrification capacity. An intermediate clarification step may be applied to reduce solids load to the nitrification trickling filter. This type of set-up produces a very high specific nitrification rate with the lowest levels of ammonia concentrations in the effluent. Denitrification Over the past decade research has proven that anoxic trickling filters are a reliable process for denitrification. The denitrification process requires an environ- ment where bacteria have no other source for the oxygen than the NO3 in the influent. In order to create high demand for oxygen, bacteria should be supplied with a carbon source (bacteria needed to oxidize C to CO2). To enable even shallow filters to denitrify ef- ficiently the filter media should be sealed against atmospheric oxygen, forcing the biomass to use nitrate (NO3) within the effluent stream for the supply of oxygen. Trickling filters can also be built in a series to allow for BOD removal and nitrifica- tion to be combined with an anoxic system to enable integrated denitrification. Existing trickling filter systems can also be adapted to provide denitrification. The normal ventilation system, which is usually located at base of the filter, can be eliminated to simplify design of the media supports and filter walls. To achieve the best results, a denitrification trickling filter should be placed upstream, where the BOD is the highest. • The Media Cross flow media ● Plastic, cross flow media is made from sheets corrugated at a 60° angle. The liquid flowing downward is split at each cross point thereby creating the maximum number of mixing points. For example, 150m2/m3 specific surface area media provides 25,000 mixing points/ m3 to optimise the mass transfer of oxygen into water across the filter. The structure ensures that applied flow goes through a continual splitting process at each point of contact between the opposite downward sloping corrugations of adjacent sheets in each media block. The cross flow configuration ensures distribution of the flow prevent- ing the effluent from choosing preferen- tial, short circuit paths Hybrid media ● Hybrid media is made from sheets of different flute sizes or cross flow sheets combined with vertical sheets. This allows optimisation of the specific surface area to allow for particular solids concentrations and more closely suit the applied loading rate. Vertical flow media ● Vertical flow media has vertical chan- nels which redistribute the liquid flow only at module interfaces. The vertical flow structure enables superior bio-solids flushing action which prevents solids 'plugging' in high rate (roughing filter) applications. • Energy usage ● Trickling filters do not need large power-hungry aeration blowers, return sludge pumps or other electrical equipment like suspended growth systems such as activated sludge and sequencing batch reac- tors. Research has shown that trick- ling filters consume around half the energy (at around 0.21 kwh per kg per kg of BOD removed) needed to power an activated sludge plant (ASP). ● Electrical power consumption needed to remove 100kg/day (over 25 year plant lifetime) ● Trickling filter plant: c. 233,000kWh of electrical power ● A coarse bubble system up to 1,900,000kWh (TF x 8) ● Fine bubble aeration system up to 3,300,000kWh (TF x 14)* ● Studies show that a gravity-fed trickling filter system (including construction and 25 year operation) emits 10% less carbon than an ASP plant. If the system is pumped, the carbon footprint is typically 30% less than an ASP set-up. More than 90% of the carbon emitted by a trickling filter instal- lation results from day-to-day operations – e.g. pumping the feed up to the filter and recycling water. If the feed-to-filter is driven by grav- ity, only the recycling pump will be required. * The power figures quoted take into account blower, feed, RAS, and SAS pumps and assume that all other services such as primary and FST drives are common to both systems.

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