Nutrient removal is one of the primary goals of wastewater treatment and large amounts of ammonia are present throughout the wastewater treatment process. Conventional ammonia removal technologies are energy intensive and do not result in recoverable forms of the nutrient. Anaerobic dewatering side-streams are the liquid recovered during the biosolids dewatering processes following anaerobic digestion. The dewatering side-streams contain high concentrations of ammonia (~1000 mg/L NH4-N) making them an excellent candidate for resource recovery technologies. In this study electrodialysis (ED) was investigated for ammonia (NH4-N) recovery from anaerobic dewatering side-streams with an emphasis on fouling and scaling control on ion exchange membranes (IEMs).
The experimental set-up consisted of 3 bench-scale electrodialyzers operating in parallel. The dewatering side-stream (centrate) was collected directly from centrifuges at a local WWTP and pretreated using a 0.3-mm screen. Electrodialyzer operation over 2.25 hrs achieved 95% NH4-N removal and the ammonia separation rate was slowed down by the concentration gradient between concentrate and diluate streams. A combined 269 hrs of operation during fouling experiments showed that electrodialysis (ED) performance decreased over time due to IEM fouling and thus clean-in-place (CIP) procedures was conducted every 60-120 hrs to restore the ED effectiveness. The two stage CIP procedures consisted of a NaCl Clean (5% NaCl, 2 hrs recirculation) and an Acid Clean (5% v/v HCl, 2 hrs recirculation). The NaCl Clean targeted organic fouling and the Acid Clean removed scales that precipitated on the IEMs. CIP procedures were able to recover 84-90% of the initial separation efficiency, the permanent loss in separation efficiency indicating that a portion of IEM fouling (10-16%) is irreversible. The higher applied voltage condition (7.5 V) showed faster fouling rates compared to low voltage conditions (4.5 V), while the degree of irreversible fouling was independent of the applied voltage. Organic fouling and inorganic scaling were individually quantified during CIP procedures using electrochemical impedance spectroscopy (EIS). While both fouling and scaling contributed significantly to the overall increase in the IEM stack resistance (63% scale formation, 37% organic fouling), inorganic scaling was found to play a more important role in reducing the separation rate in ED. ICP and SEM-EDS analysis identified the scale that formed on the surface of the IEMs as mostly of CaCO3 precipitation with smaller amounts of struvite. This finding indicates that the pretreatment of dewatering side-streams should be more focused on removing divalent cations (Ca2+ and Mg2+), but also still consider organic foulant removal for its treatment in ED. Since organic fouling primarily affects anion exchange membranes (AEMs), the impacts of fouling were investigated on two types of AEMs (AR908, AR204). Fouling experimentation showed minor differences in current density and separation efficiency over 269 hrs of operation, with AR204 AEMs showing signs of worse irreversible fouling. Particle size analysis of centrate suggested that large suspended particles could obstruct electrodialyzer chambers. Screening pretreatment (0.3 mm) effectively removed large particles and mitigated clogging issues without having to increase intermembrane distances.
The experimental results suggest that ED is a promising technology for recovering ammonia from nutrient rich wastewaters. ED was able to efficiently achieve high levels of ammonia separation from centrate, while fouling was shown to be manageable using CIPs at reasonable intervals. Overall ED was shown to be an effective way to recover ammonia from dewatering side-streams, but pretreatments targeting scaling and organic fouling could better mitigate performance losses due to fouling and further improve the process. / Thesis / Master of Applied Science (MASc)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/24348 |
| Date | 06 1900 |
| Creators | Alex, Andrew |
| Contributors | Kim, Younggy, Civil Engineering |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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