Ozonation of Municipal Wastewater for Water Reuse

Abstract

Water is essential for life in all the levels: humans, animals and plants depend on it for their existence. The unsustainable growth, the changes in the consumption pattern and the climate change have positioned water resources under pressure. In this scenario where the water quality and quantity are a worldwide concern, research and development have analyzed and generated various emerging technologies that can promote the use of alternative sources of water.

Commonly, the Municipal WWTPs are organized in four stages that include physical, chemical and biological processes. Wastewater treatment plants (WWTPs) will have an important role in the future eco-cities since will provide energy by means of a system characterized by the smallest possible ecological footprint. However, several ecological effects have been observed downstream of WWTP outfalls, probably due to the incomplete removal of pollutants by the conventional WWTPs. The analysis and understanding of the pollutants fate in Conventional WWTPs is fully necessary to create measures to reduce their release in the environment.

Even though, conventional WWTPs are designed to treat solid wastes, suspended solids and biodegradable dEfOM, many micropollutants are also affected by the treatment train. Hence, half of the micropollutant load is eliminated either by sorption to the sludge, biological degradation, volatilization and abiotic degradation. Despite this fact, some hydrophilic compounds are neither adsorbed into the sludge nor eliminated in the discharge effluents. Consequently, some of these micropollutants can end up in the aquatic environment generating negative effects, including long-term and short-term toxicity.

In the problematic water pollution scenario mentioned before, advanced oxidation processes (AOPs) emerge as a possible alternative to treat the biologically persistent wastewater improving water quality and therefore restoring the aquatic environment. These processes degrade organic pollutants by forming hydroxyl radicals (OH·) which are highly reactive and non- selective. Nowadays, AOPs include also the processes that involve other radicals as sulfate radicals (SO4 ·).

Ozone application is used in wastewater, disinfection and air treatment to minimize the pollution. This process has two main strengths: on one hand, the strong oxidant potential and secondly, the lack of residues after its application. Ozone can react directly, via molecular pathway or indirectly via hydroxyl radical.

In this work, ozone has been applied to different non-conventional points of the treatment line, to check if its action could promote the enhancement of the whole treatment. Thus, it has been applied at the outlet of the primary effluent leading us to an improvement in the water quality parameters and in the removal of micropollutants. Moreover, other significant parameters for ozone application as the ozone demand and mass transfer have been studied.

Afterwards, the study was focus in the application of ozone on the activated sludge matrix. In this case, ozone showed good performance too, improving the settleability, increasing the solubility of sludge and eliminating micropollutants in both phases (sludge and supernatant).

Finally, the combination of ozone application with biological treatments was tested. Thus, ozone was applied to the primary effluent which was lately treated by an aerobic biological treatment.

In this case, good performances were observed at the level of micropollutants. Lately, ozone was applied to the conventional activated sludge matrix which was subsequently introduced in an anaerobic process to check the enhancement of biogas production. In this case, only two ozone doses showed better performances than the initial sludge without ozone pretreatment. Biodegradability and acute toxicity was studied for the primary effluent before and after ozonation, showing an improvement when the transfer ozone dose increased.

L’aigua es essencial per la vida en tots els nivells: humans, animals i plantes depenen d’ella per la seva pròpia existència. El creixement insostenible, els canvis en el patró de consum i el canvi climàtic han situat els recursos hídrics sota pressió. En aquest escenari on la qualitat de l’aigua i la quantitat són una preocupació mundial, la recerca i el desenvolupament analitzen actualment les tecnologies que promoguin el ús alternatiu dels recursos hídrics.

En l’escenari de la problemàtica de l’aigua, els processos d’oxidació avançada emergeixen com a possible alternativa per tractar els compostos recalcitrants que trobem en l’aigua, millorant la qualitat de l’aigua i per tant, restaurant el medi aquàtic. Aquests processos, degraden els contaminants orgànics formant radicals hidroxils (·OH) que són altament reactius i no són selectius.

L’ozó s’utilitza en el tractament d’aigües residuals, desinfecció i tractament de l’aire per minimitzar la contaminació. Aquest procés té dos punts forts: per una banda, el fort potencial oxidant i en segon lloc, la manca de residus després de la seva aplicació. L’Ozó pot reaccionar directament, via molecular o indirectament via el radical hidroxil.

En aquest projecte, l’ozó s’ha aplicat a diferents punts de la cadena de tractament, per tal de comprovar si la seva acció pot promoure una millora en el tractament global. Així, s’ha aplicat a l’efluent de la sortida del tractament primari millorant els paràmetres de l’efluent primari tant a nivell de la qualitat de l’aigua i de l’eliminació dels microcontaminants. A més, altres paràmetres importants relacionats amb l’aplicació de l’ozó i la demanda d’ozó han estat estudiats.

Després, l’estudi es va centrar en l’aplicació de l’ozó en una matriu de Fangs Activats. En aquest cas, ozó va mostrar bons resultats millorant la sedimentació, incrementant la solubilitat dels fangs i l’eliminant els microcontaminants.

Finalment, l’ozó es va combinar amb tractaments biològics, aeròbics i anaeròbics. Bones eliminacions es van observar a nivell de microcontaminants. Tot i així, només dues dosis van afavorir una millora en la producció del biogàs.