The application of oxidation processes in groundwater treatment of the eastern Slavonian region

Cilj ovog rada je istraživanje primjenjivosti ozona u obradi bunarskih voda s povišenim sadržajem prirodnih organskih tvari (POT), što je između ostalog, zajednička karakteristika podzemnih voda istočne Slavonije. Za potrebe ispitivanja koristila se bunarska voda koja se u svrhu pripreme pitke vode grada Osijeka crpi s izvorišta Vinogradi. U suvremenim tehnološkim procesima pripreme pitke vode ozon se primjenjuje za dezinfekciju vode, uklanjanje anorganskih tvari, razgradnju organskih tvari i za poboljšanje procesa koagulacije i flokulacije. U ovom radu naglasak je stavljen na istraživanje utjecaja ozonizacije na razgradnju prirodnih organskih tvari, stabilnost i taloženje koloidnih čestica, porast biorazgradivosti te na uklanjanje arsena iz vode. U tu svrhu izrađen je laboratorijski eksperimentalni sustav koji se sastoji od generatora ozona, reaktora za ozonizaciju, termostatirajuće kupke i mjerne opreme za praćenje sadržaja ozona. Rezultati laboratorijskih ispitivanja su pokazali malo smanjenje sadržaja otopljenog organskog ugljika, značajno smanjenje UV apsorbancije kod 254 nm te porast biorazgradivosti produkata ozonizacije. Kao indeks biorazgradivosti izabran je BPK/KPK omjer. Porast biokemijske potrošnje kisika (BPK) nakon ozonizacije upućuje na veću sklonost produkata prema biorazgradivosti. Prema tome, porast BPK/KPK omjera nakon ozonizacije ukazuje na poboljšanu biorazgradivost zbog formiranja razmjerno lako biorazgradivih niskomolekularnih organskih spojeva – aldehida, karboksilnih kiselina i ketokiselina. Povrh toga, u laboratorijskim uvjetima su određivane adsorpcijske izoterme za četiri različite vrste aktivnog ugljena u granulama (AUG). Izabran je Freundlichov model za opisivanje adsorpcije prirodnih organskih tvari na AUG-u i procjenu efikasnosti njihovog uklanjanja u realnim postrojenjima. S ciljem potvrđivanja preliminarnih rezultata dobivenih u laboratorijskim uvjetima, izrađen je pilot uređaj u Vodovodu Osijek koji se sastoji od generatora ozona, reaktora za ozonizaciju i dva tlačna filtra u paralelnom radu – sa i bez prethodne ozonizacije, ispunjena s istom vrstom aktivnog ugljena. Istraživanje na pilot uređaju je pokazalo da ozonizacija prirodnih organskih tvari utječe na porast njihove biorazgradivosti te da se mogu naknadnom biološkom filtracijom ukloniti iz vode. S obzirom da postoji velika vjerojatnost da će se u bliskoj budućnosti maksimalno dozvoljena koncentracija (MDK) ukupnog arsena u pitkoj vodi značajno smanjiti, posebna pozornost je posvećena uklanjanju arsena iz vode. Rezultati pilot ispitivanja su pokazali da učinkovitost uklanjanja arsena ovisi o sadržaju željeza u vodi. Da bi se potaknuli procesi adsorpcije arsena i koprecipitacije sa željezovim hidroksidom, dozirao se željezov klorid. Za koncentraciju arsena u vodi od približno 50 μg/l, doziranjem 1 mg Fe(III)/l postiglo se uklanjanje arsena do koncentracije ispod 10 μg/l.The general objective of this work was to examine the applicability of ozone for treating groundwater containing significant amounts of natural organic matter (NOM). The water, used in this work, is taken from the eastern Slavonijan region, and is originally used for preparation of drinking water for the city of Osijek. Ozonation is used in drinking water treatment to accomplish numerous treatment goals, including disinfection, removal of inorganic species, degradation of organic matter and as an aid to the coagulation-flocculation process. The emphasis of this work was to study the influence of ozonation on the degradation of natural organic matter, particle destabilization, and increase in the biodegradability of ozonation products as well as arsenic removal from water. For this purpose, an experimental set up was designed for laboratory experiments. It consisted of an ozone generator, contacting tank, thermostat and ozone measuring device. The results of the laboratory experiments have shown a small decrease in dissolved organic carbon and a substantial decrease in UV absorbance at 254 nm as well as an increase in biodegradability of ozonation products. BOD to COD ratio has been chosen as a biodegradability index. An increase of the BOD value due to the pretreatment with the ozone indicates greater tendency to biodegradation. Therefore, an increase in the BOD to COD ratio after ozonation indicates improved biodegradability due to an increase of lower molecular weight organic compounds such as aldehydes, carboxylic acids and keto acids that are more easily biodegradable. In addition, adsorption capacities of four different granular activated carbon (GAC) types were determined from isotherm experiments. The Freundlich model was selected to describe the adsorption of natural organic matter onto GAC and the prediction of their removal in larger-scale GAC applications. In order to confirm preliminary results obtained from laboratory experiments and to make a general conclusion, a pilot plant was designed. It consisted of an ozone generator, ozone contact column and two GAC filters. Through one of the GAC filters passed ozonated water and through another passed directly nonozonated water. The pilot study showed that ozonation of natural organic matter prior to adsorption onto GAC is beneficial because of the formation of biodegradable organic compounds that can be easily removed by biologically activated carbon filtration. Regarding the possibility that the maximum contaminant level (MCL) for total arsenic in drinking water will be substantially reduced in the near future, special attention has been devoted to arsenic removal from water. The results obtained from the pilot study indicate that arsenic removal efficiency depends on the amount of iron content in water. In order to encourage the adsorption and co-precipitation of arsenic with iron hydroxide precipitates, ferric chloride was added. For an arsenic concentration of approximately 50 μg/L, 1 mg/L Fe(III) was required to reduce arsenic concentration to less than 10 μg/L