Water and Effluent Treatment Magazine
Issue link: https://read.utilityweek.co.uk/i/283328
26 WET NEWS MARCH 2014 The challenge: Water Treatment INSIGHT Biological iron removal has many advantages over conventional treatment, and has proven 'invaluable'. A solution: Biological treatment reduces iron T he bacteria that occur nat- urally in groundwater re- sponsible for the biologi- cal oxidation of ferrous iron include stalked and filamentous bacteria. This biological oxida- tion takes place either by pri- mary intracellular oxidation through enzymes, or secondary extracellular oxidation due to catalytic action of polymers ex- creted by the iron bacteria. The oxidised precipitates are typically in the form of slightly hydrated iron oxides, which are stored in the secretions of these bacteria, stalks or sheaves. These are more compact that the precipitates formed in conventional treatment and result in lower clogging head in the filters and sludge easier to thicken. Various sites have been reviewed which characterise the raw water quality in terms of dissolved oxygen, more regu- larly measured than redox potential, and pH. The lower pH, the higher the allowable dissolved oxygen, and in the case of East Ruston, no oxygen addition is required at all. On three of the works, simul- taneous ammonia removal takes place in the biological iron stage (Walpole Pettistree and Powdermill) and in the biologi- cal manganese stage (Saints Hill) where it is observed that the bottom half of the filter medium is black due to the manganese. Hence, when the Redox con- ditions are satisfied, biological ammonia removal can also occur, and this is best defined during pilot trials. All these plants com- prise pressurised filters for biological iron removal and wash water recovery, where practised, is returned down- stream of the biological iron stage. Powdermill WTW is a bore- hole sourced treatment works in Kent, located near Tonbridge. The treatment capacity is 4.5Mld, and oxygen is provided for the biological iron stage by direct air injection and set at 10% saturation. The ammonia reduction across the iron filters is about 65% on a 95%ile basis and 90% on average. Biological iron plants are very robust and start up very quickly and the results from Walpole commissioning show this.Walpole WTW is a borehole sourced treatment works in Suf- folk, located near Halesworth. The original treatment com- prised aeration, single stage fil- tration followed by chlorination. The upgraded works increases the treatment capacity from 3.9 to 6.13Ml/d with the introduction of a new borehole. The detailed design by MWH involved the installation of new pressure filters designed for both biological iron and ammo- nia removal, with a cascade aeration system. The existing filters provide a dedicated phys- ico-chemical stage for manga- nese removal. The plant can be stood down for up to a week without dam- age to the bacteria and takes less than 24h to provide water quality below 50µg/l. The new bore has a concen- tration of ammonia around 0.7mg/l, and it was found ini- tially when this water was intro- duced to the system, a small amount of ammonia broke through the primary filters. This was removed with addi- tional chlorine until the biologi- cal ammonia process adjusted to the increased concentration. The process of simultaneous removal of iron and ammonia does not respond well to sudden change in concentrations of ammonia. The filtered water total iron is about 0.02mg/l cor- responding to a turbidity of around 0.1 NTU. Tests during commissioning indicated a dissolved oxygen concentration of 3 to 5mg/l required for simultaneous iron and ammonia removal. It was recognised that the conditions provided are sub- optimal for both the iron and ammonia processes, but are ade- quate for both to work satisfacto- rily at conservative filtration rates. It was noted that the media size increases markedly over time, despite apparently good filter washing. The media coat- ing has been analysed as iron and hardness, and this phenom- enon has also been seen in South East Water and may be attributable to the high hard- ness of the borehole water. Pettistree WTW is a borehole sourced treatment works near Wickham Market designed for a flow of 8.26Mld. Raw Water Analysis The original design comprised gravity filters equipped with manganese dioxide for manga- nese removal preceded by spray aeration. Due to the high iron concen- tration, and following pilot plant trials by Biwater (now MWH Treatment) biological iron pressure filters were installed upstream of the grav- ity filters as a roughing stage, with in line air injection. Typically 0.1mg/l iron con- centration is obtained at the outlet of the roughing filters and 0.005mg/l in the final water Based on this review led by MWH, water companies can now get advice on optimising their iron removal plants. The first step is an analysis of the raw water that will give a good idea whether this process is applicable and possibly fol- lowed by pilot tests As can be seen from the above, biological treatment offers an efficient high rate pro- cess for reducing iron particu- larly when a separate • The commissioning results with total iron reducing to approximately 70µg/l from 620µg/l with 42h manganese stage is required. The combination of these process steps with biological iron typi- cally in the range of 20m/h for iron concentrations at around 2mg/l and manganese removal at similar velocities results in sub- stantial cost savings as a conven- tional physico- chemical plant (sand and catalytic media) may operate only at a combined veloc- ity of 6m/h. "Overall biological iron removal is a very good process. For the raw water iron concentra- tions we experience, it offers high filtration rates compared to phys- ico-chemical alternative. It is currently the preferred treatment process in the Suffolk area for iron removal in bore- holes," says Mike Kirk, of Essex & Suffolk Water, and according to Peter Barratt, of Anglian Water:"It is a much underused method of treating water that is high in iron, and the process works efficiently compared with clarification as an initial removal stage." Jeanette Sheldon, of South East Water concludes: "South East Water have a number of bore- holes with very high concentra- tions of naturally occurring iron (and manganese). The biological removal process is invaluable in treating such raw water and pro- vides a number of key advantages over other processes as discussed in this paper." n Stewart Tattersall is a senior princi- pal Process engineer at MWH. THE CONCEPT • High filtration velocities – up to 50m/h depending upon the iron concentration • No use of oxidation chemicals and minimal oxygen requirements (OPEX savings) • Greater iron removal capacity up to 5kg/m2 and hence greater intervals between washes • Higher sludge settling rates, therefore easier to thicken • Does not complex with organic substances or silica • Can use raw water for backwashing • Can simultaneously remove ammonia thereby obviating breakpoint chlorination (within certain parameters) NEED TO KNOW 1 Iron in the soluble form is generally removed from groundwater by physico-chemical oxidation 2 Biological iron removal is an alternative process that has been in operation for the past 20 years at 15 or so plants in the eastern part of the UK 3 Biological iron plants are very robust and start up very quickly 4 Biological treatment offers an efficient high rate process for reducing iron particularly when a separate manganese stage is required 5 Typically 0.1mg/l iron concentration is obtained at the outlet of the roughing filters and 0.005mg/l in the final water THE VERDICT • It is a much underused method of treating water that is high in iron, and the process works efficiently compared with clarification as an initial removal stage. • The biological removal process is invaluable in treating such raw water and provides a number of key advantages over other processes as discussed in this paper. "It is currently the preferred treatment process in the Suffolk area for iron removal in boreholes" Mike Kirk, Essex & Suffolk Water MARCH 2014 WET NEWS 27