Βιολογική επεξεργασία λυμάτων με χρήση μεμβρανών

Abstract

Scarce water resources in several parts of the world have led to an increased interest in wastewater reuse applications and have promoted the development and application of advanced wastewater treatment technologies. The Membrane Bioreactor (MBR) system is an advanced wastewater treatment system which has been adopted the last years for the biological treatment of wastewater instead of the conventional system of Activated Sludge (AS). MBR systems combine biological treatment and membrane filtration for the secondary treatment of wastewater. The membranes are employed in order to separate the final effluent from sludge. Therefore, this system does not employ a secondary sedimentation tank for effluent clarification and consequently the problems of poor sludge settling due to sludge bulking or due to sludge deflocculation are not encountered. Moreover, MBRs achieve a superior effluent quality that is comparable or even superior to tertiary treatment systems as the membranes are an absolute barrier to suspended solids and to colloidal matter. The adoption of MBRs can lift the barriers for the wider adoption of wastewater reuse, since the final effluent meets the stringiest reuse guidelines for unrestricted irrigation, provided that some disinfection is ensured. This is particularly important for countries facing significant water shortage problems such as Greece. The main operational problem associated with MBR systems is that of membrane fouling, which is defined as the gradual deposition of particles at the membrane surface and interior. As the system operates, the transmembrane pressure (TMP) gradually increases for given membrane flux due to membrane fouling. Once the TMP has reached a certain peak value the membrane module must be removed and must be chemically cleaned in order to restore its permeability. A variety of methods are employed during the system operation to limit fouling; these include backwash, coarse bubble-aeration and chemical cleaning at low doses inside the biological reactor. Coarse bubble aeration significantly increases the system's operational cost. However, even with the adoption of these methods fouling gradually builds up and intensive chemical cleaning is required in which case the module is removed from the biological reactor and is placed in a suitable chemical cleaning solution for several hours. The latter method is the most effective for restoring membrane permeability, but shortens the life-time of the membrane and is thus only periodically employed. As it is evident, membrane fouling increases the operational cost of the system. Inevitably, much research has focused on understanding membrane fouling and its mechanisms, on determining the factors that mostly affect it and on determining cost-effective methods for its limitation. Fouling is a highly complex phenomenon which is influenced by several parameters; these can be grouped in four main categories: (1) the membrane properties, (2) the biomass characteristics, (3) the operating conditions and (4) the influent wastewater. The membrane characteristics and the biomass properties directly impact on membrane fouling, while the operating parameters and the influent wastewater mainly influence fouling indirectly through their impact on biomass properties. The main research question that this thesis addresses is the effect of the solids retention time (SRT) and of the sludge properties on membrane fouling. A series of important issues are investigated. These are: • The impact of the solids retention time on biomass properties and consequently on membrane fouling. • The impact of biomass properties on long-term and on short-term membrane fouling. • The dominant fouling mechanisms in both long-term and short-term membrane fouling. • The determination of the dominant fraction of biomass and of extracellular polymeric substances for the solids retention times under investigation. • The impact of the addition of chemical and physical conditioners to sludge in order to minimize membrane fouling. • The heterotrophic and autotrophic biomass activity. • The MBR long-term biological performance. The long-term membrane fouling was investigated through the operation of a pilot scale MBR system for approximately 2,5 years at SRT = 10, 15, 20 and 33 days, while the short-term membrane fouling was investigated with the 60-minute filtration experiments conducted in a batch ultrafiltration system. […]

[…] Η παρούσα διδακτορική διατριβή έχει ως βασικό στόχο την εξέταση της επίδρασης των λειτουργικών παραμέτρων και των χαρακτηριστικών της βιομάζας στην έμφραξη των μεμβρανών. Η βασική λειτουργική παράμετρος που διερευνάται είναι ο χρόνος παραμονής των στερεών στο σύστημα (θc), ενώ εξετάζεται η επίδραση μιας σειράς ιδιοτήτων της ιλύος στη μακροχρόνια έμφραξη, όπως είναι τα εξωκυτταρικά πολυμερή, τα κολλοειδή, η συγκέντρωση του ανάμικτου υγρού, η διηθησιμότητα και η καθιζησιμότητα της ιλύος. Επιπλέον, στα πλαίσια της παρούσας διατριβής εξετάζεται η επίδραση των χαρακτηριστικών της βιομάζας στη βραχυχρόνια έμφραξη των μεμβρανών. Η μακροχρόνια έμφραξη εξετάζεται με τη συνεχή λειτουργία πιλοτικού συστήματος MBR για διάστημα 2,5 περίπου ετών σε θc = 10, 15, 20 και 33 ημέρες. Η βραχυχρόνια έμφραξη εξετάζεται σε πρότυπο, εργαστηριακό αντιδραστήρα μεμβρανών υπερδιύλισης διακοπτόμενου έργου με διήθηση ιλύος για 60 λεπτά. Παράλληλα, με την εφαρμογή κατάλληλων μοντέλων διήθησης εξετάζονται οι κυρίαρχοι μηχανισμοί έμφραξης, τόσο για τη μακροχρόνια όσο και για τηβραχυχρόνια λειτουργία και εξετάζεται κατά πόσο οι μηχανισμοί αυτοί είναι ταυτόσημοι. Επίσης, εξετάζονται τα κυρίαρχα κλάσματα του ανάμικτου υγρού και των εξωκυτταρικών πολυμερών για τις υπό διερεύνηση λειτουργικές συνθήκες, καθώς και η επίδραση της προσθήκης συγκεκριμένων ουσιών (φυσικών ορυκτών, κροκιδωτικών και κατιονικού πολυμερούς) στη βιομάζα με στόχο τη μείωση της έμφραξης. Η εξέταση της επίδρασης των προσθέτων στη μείωση της έμφραξης πραγματοποιείται στον εργαστηριακό αντιδραστήρα μεμβρανών υπερδιύλισης. Τέλος, εξετάζεται η δραστηριότητα της ετεροτροφικής και αυτοτροφικής βιομάζας καθώς και οι γενικότερες βιολογικές λειτουργίες του συστήματος MBR. […]