Increasing urine nitrification performance with sequential membrane aerated biofilm reactors.

This study aimed to investigate whether separating organics depletion from nitrification increases the overall performance of urine nitrification. Separate organics depletion was facilitated with membrane aerated biofilm reactors (MABRs). The high pH and ammonia concentration in stored urine inhibited nitrification in the first stage and therewith allowed the separation of organics depletion from nitrification. An organics removal of 70 % was achieved at organic loading rates in the influent of 3.7 gCOD d-1 m-2. Organics depletion in a continuous flow stirred tank reactor (CSTR) for organics depletion led to ammonia stripping through diffused aeration of up to 13 %. Using an MABR, diffusion into the lumen amounted for 4 % ammonia loss only. In the MABR, headspace volume and therefore ammonia loss through the headspace was negligible. By aerating the downstream MABR for nitrification with the off-gas of the MABR for organics depletion, 96 % of the ammonia stripped in the first stage could be recovered in the second stage, so that the overall ammonia loss was negligibly low. Nitrification of the organics-depleted urine was studied in MABRs, CSTRs, and sequencing batch reactors in fed batch mode (FBRs), the latter two operated with suspended biomass. The experiments demonstrated that upstream organics depletion can double the nitrification rate. In a laboratory-scale MABR, nitrification rates were recorded of up to 830 mgNL-1 d-1 (3.1 gN m-2 d-1) with ambient air and over 1500 mgNL-1 d-1 (6.7 gN m-2 d-1) with oxygen-enriched air. Experiments with a laboratory-scale MABR showed that increasing operational parameters such as pH, recirculation flow, scouring frequency, and oxygen content increased the nitrification rate. The nitrification in the MABR was robust even at high pH setpoints of 6.9 and was robust against process failures arising from operational mistakes. The hydraulic retention time (HRT) required for nitrification was only 1 to 2 days. With the preceding organics depletion, the HRT for our system requires 2 to 3 days in total, whereas a combined activated sludge system requires 4 to 8 days. The N2O concentration in the off-gas increases with increasing nitrification rates; however, the N2O emission factor was 2.8 % on average and independent of nitrification rates. These results indicate that the MABR technology has a high potential for efficient and robust production of ammonium nitrate from source-separated urine.

Comparing the adsorption of micropollutants on activated carbon from anaerobically stored, organics-depleted, and nitrified urine.

Separate collection and treatment of urine optimizes nutrient recovery and enhances micropollutant removal from municipal wastewater. One typical urine treatment train includes nutrient recovery in three biological processes: anaerobic storage, followed by aerobic organics degradation concurrently with nitrification. These are usually followed by activated carbon adsorption to remove micropollutants. However, removing micropollutants prior to nitrification would protect nitrifiers from potential inhibition by pharmaceuticals. In addition, combining simplified biological treatment with activated carbon adsorption could offer a cheap and robust process for removing micropollutants where nutrient recovery is not the first priority, as a partial loss of ammonia occurs without nitrification. In this study, we investigated whether activated carbon adsorption could also take place between the three biological treatment steps. We tested the effectiveness of micropollutant removal with activated carbon after each biological treatment step by conducting experiments with anaerobically stored urine, organics-depleted urine, and nitrified urine. The urine solutions were spiked with 19 pharmaceuticals: amisulpride, atenolol, atenolol acid, candesartan, carbamazepine, citalopram, clarithromycin, darunavir, diclofenac, emtricitabine, fexofenadine, hydrochlorothiazide, irbesartan, lidocaine, metoprolol, N4-acetylsulfamethoxazole, sulfamethoxazole, trimethoprim, venlafaxine, and two artificial sweeteners, acesulfame and sucralose. Batch experiments were conducted with powdered activated carbon (PAC) to determine how much activated carbon achieve which degree of micropollutant removal and how organics, pH, and speciation change from ammonium to nitrate influence adsorption. Micropollutant removal was also tested in granular activated carbon (GAC) columns, which is the preferred technology for micropollutant removal from urine. The carbon usage rates (CUR) per person were lower for all urine solutions than for municipal wastewater. The results showed that organics depletion would be needed when micropollutant removal was the sole aim of urine treatment, as the degradation of easily biodegradable organics prevented clogging of GAC columns. However, CUR did hardly improve with organics-depleted urine compared to stored urine. The lowest CUR was achieved with nitrified urine. This resulted from the additional organics removal during nitrification and not the lower pH or the partial conversion of ammonium to nitrate. In addition, we showed that the relative pharmaceutical removal in all solutions was independent of the initial pharmaceutical concentration unless the background organics matrix changed considerably. We conclude that removal of micropollutants in GAC columns from organics-depleted urine can be performed without clogging, but with the drawback of a higher carbon usage compared to removal from nitrified urine.

High content of low molecular weight organics does not always affect pharmaceutical adsorption on activated carbon: The case of acetate, propionate and ethanol in source-separated urine.

Adsorption on activated carbon is a common process to remove pharmaceuticals in wastewater treatment. Activated carbon adsorption is usually applied to wastewater with a low content of biological degradable organics, i.e. after biological treatment. Especially low molecular weight (LMW) compounds are known to compete with pharmaceuticals for adsorption sites. The goal of this study was to test the hypothesis that biological treatment is necessary for efficient pharmaceutical removal. Source-separated urine after anaerobic storage (anaerobically stored urine) and after aerobic biological removal of organics without nitrification (organics-depleted urine) were used in this study. In anaerobically stored urine 60% of the organic compounds were LMW organics, of which about 40% were acetate and propionate. 74% of the DOC and 100% of acetate and propionate were removed during aerobic biological treatment. To investigate the effect of the organic compounds on pharmaceutical removal, sorption experiments with 19 spiked pharmaceuticals and one artificial sweetener were conducted with powdered activated carbon. Ethanol, another LMW organic, was included in the study, as it is regularly used for pharmaceutical spiking thereby strongly increasing the DOC content. The experiments showed that the adsorption of the pharmaceuticals and the sweetener were hardly affected by the easily biodegradable LMW organics or ethanol. Therefore, it was concluded that biological pre-treatment is not necessary for efficient pharmaceutical adsorption. Since acetate, propionate and ethanol contribute substantially to the DOC content but do not absorb UV light, the latter is recommended as indicator for pharmaceutical removal in solutions with high contents of biodegradable LMW organics.