Performance and onsite regeneration of natural zeolite for ammonium removal in a field-scale non-sewered sanitation system

Abstract

Natural zeolite clinoptilolite was used as the primary ammonium removal method from the permeate of an anaerobic membrane bioreactor (AnMBR) treating high-strength blackwater generated from a community toilet facility. This zeolite-based nutrient capture system (NCS) was a sub-component of a non-sewered sanitation system (NSSS) called the NEWgenerator and was field tested for 1.5 years at an informal settlement in South Africa. The NCS was operated for three consecutive loading cycles, each lasting 291, 110, and 52 days, respectively. Both blackwater (from toilets) and blackwater with yellow water (from toilets and urinals) were treated during the field trial. Over the three cycles, the NCS was able to remove 80 ± 28%, 64 ± 23%, and 94 ± 11%, respectively, of the influent ammonium. The addition of yellow water caused the rapid exhaustion of zeolite and the observed decrease of ammonium removal during Cycle 2. After Cycles 1 and 2, onsite regeneration was performed to recover the sorption capacity of the spent zeolite. The regenerant was comprised of NaCl under alkaline conditions and was operated as a recycle-batch to reduce the generation of regenerant waste. Modifications to the second regeneration process, including an increase in regenerant contact time from 15 to 30 h, improved the zeolite regeneration efficiency from 76 ± 0.7% to 96 ± 1.0%. The mass of recoverable ammonium in the regenerant was 2.63 kg NH₄-N and 3.15 kg NH₄-N after Regeneration 1 and 2, respectively. However, the mass of ammonium in the regenerant accounted for only 52.8% and 54.4% of the estimated NH₄-N originally sorbed onto the zeolite beds after Cycles 1 and 2, respectively. The use of zeolite clinoptilolite is a feasible method for ammonium removal by NSSS that observe variable nitrogen loading rates, but further research is still needed to recover the nitrogen from the regenerant waste.

Keywords: Ammonium; Non-sewered sanitation; Resource recovery; Sorption; Wastewater; Zeolite.

Graphical abstract

Fig. 1

Flow diagram of the NCS during A) normal field trial operations and B) the regeneration process.

Fig. 2

Timeline of events during the field trial. Circles represent time points when the influent media changed; squares represent scheduled shutdown periods when UKZN staff were on summer holiday; and triangles represent the start and end of regeneration events. The operational time is divided into three cycles: Cycle 1 (green shaded region), Cycle 2 (pink shaded region), and Cycle 3 (blue shaded region). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Concentration of nitrogenous species (total nitrogen, ammonium, and nitrate) measured across the entire field trial. The operational time is divided into three cycles: Cycle 1 (green shaded region), Cycle 2 (pink shaded region), and Cycle 3 (blue shaded region). Note: Summer shutdown (SS) and Regeneration events (Regen). TN data for the NG influent and after Chlorination are reproduced from Shyu et al. (2021). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 4

A) Ammonium loading rate into the zeolite beds (unfilled circle) and percent ammonium removal by the NCS (black circle) and B) ammonium sorption by zeolite beds of the NCS during the entire field trial. The laboratory estimated maximum ammonium sorption capacity of the zeolite used in this study was 16.2 g/kg.

Fig. 5

A) Electron equivalents of ammonium and sodium and B) conductivity and pH measured in the regenerate solution during Regeneration 1. Note: recirculation period (rc) and grey areas represent periods of static and/or overnight soaking only.

Fig. 6

A) Conductivity and B) pH of the influent (permeate) and effluent (NCS) of the NCS during entire field trial. No conductivity data is available for Cycle 3.

Fig. 7

Ammonium mass balance during the two regeneration events conducted during the field trial.