Nitrogen recovery from urban wastewater by means of membranes technology

Abstract

This study aims to contribute in a small way to the path of circular economy. Therefore, the contamination threat that UWW supposes is explained here as well as its potential as a source for N recovery. This recovery entails two benefits, in one hand, lower concentrations in the effluent discharged into the environment are achieved, and on the other hand, a compound suitable as fertilizer is produced. A bibliographic research is carried out in order to determine the impact of these problematics as well as existing technologies to face them. The most important treatments used nowadays for WWTPs are explained to know how these plants operate and which N recovery techniques exist. A feasibility analysis for those techniques involving membranes technology or struvite precipitation is carried out from data obtained in literature. This analysis can be divided in the study of two aspects, technical capability of the technique to be implemented in plant, and economic balance. Technical capability evaluates the similarity between the stream parameters required for each technique and the actual conditions of wastewater in WWTP as well as the efficiency of both N removal and recovery. Only techniques capable to be performed in streams existent in conventional municipal WWTPs as well as to remove enough N to produce a harmless effluent for the environment are considered suitable to implement. Economical balance takes into account benefit produced from final product’s sale, energetic costs, operational costs and chemical costs. Agronomical and environmental considerations of final products are explained since they have a direct repercussion on final fertilizers sale price and its applicability to the soils. Maresme WWTPs characteristics are presented because they are the base for implementation calculation. These plants water line and sludge line are described too. Once the most suitable techniques are determined, a subsequent and detailed study for Hollow Fibre Membrane Contactors (HFMC) and Struvite Precipitation (SP) is carried out. These processes are explained in depth and various parameters related with each technique’s implementation are calculated. Thanks to these calculation, the values of ammonia recovered, product generated, and economic considerations could be taken into account for the case of Maresme plants. Operational advantages and disadvantages for both techniques are presented too considering their implementation in Maresme WWTPs. By analysing the impact of their implementation in plant, a comparison between both techniques is carried out. It is determined that great amounts of N could be recovered using HFMC and the economic balance would be significantly high. However, this technique would present operational problems like membrane fouling. SP technique present the advantage of recovering P but its N recovery efficiency is much lower. Despite its economic balance is negative, controlled precipitation of struvite represents a great benefit for those plants affected by uncontrolled crystallisation. Finally, a combination of both techniques implementation in a sequence of SP and MC is studied since calculation results suggest that this could present synergies. Several benefits could be achieved by operating with this combination: great recoveries would be reached for both N and P, and operational problems as membrane fouling or uncontrolled struvite crystallisation would be solved or sharply reduced while economic balance would be positive