Energy and phosphorus recovery from black water

Abstract

Source-separated black water (toilet water) containing 38% of the organic material and 68% of the phosphorus in the total household waste(water) stream including kitchen waste, is a potential source for energy and phosphorus recovery. The energy recovered, in the form of electricity and heat, is more than sufficient for anaerobic treatment, nitrogen removal and phosphorus recovery. The phosphorus balance of a UASB reactor treating concentrated black water showed a phosphorus conservation of 61% in the anaerobic effluent. Precipitation of phosphate as struvite from this stream resulted in a recovery of 0.22 kgP/p/y, representing 10% of the artificial phosphorus fertilizer production in the world. The remaining part of the phosphorus ended up in the anaerobic sludge, mainly due to precipitation (39%). Low dilution and a high pH favour the accumulation of phosphorus in the anaerobic sludge and this sludge could be used as a phosphorus-enriched organic fertilizer, provided that it is safe regarding heavy metals, pathogens and micro-pollutants. Keywords anaerobic treatment, black water, biogas production, phosphorus recovery, separation at source, struvite INTRODUCTIONNew sanitation concepts are based on separation at source of household wastewater streams into grey water and black water (faeces and urine) or into grey water, urine and brown water (faeces), and have a large potential to recover the important resources energy, nutrients and water. Anaerobic treatment is regarded as the core technology for energy and nutrient recovery from source separated domestic wastewater (Otterpohl et al., 1999;Kujawa-Roeleveld and Zeeman, 2006). Previous research showed that concentrated black water (faeces and urine collected in a vacuum toilet) can be efficiently treated in a UASB (Upflow anaerobic sludge blanket) reactor at a relatively short hydraulic retention time (HRT) of 9.1 days (de Graaff et al., 2010), in a UASB septic tank at a longer HRT of 29 days (Kujawa-Roeleveld et al., 2005) or in a CSTR at an HRT of 20 days (Wendland et al., 2007). A methane production of 1.8 m 3 CH 4 per m 3 of black water can be achieved, which can be converted to 56 MJ/p/y as electricity and 84 MJ/p/y as heat by combined heat and power (CHP) (de Graaff et al., 2010). The nutrients are largely conserved in the effluent of the anaerobic treatment step. Van Voorthuizen et al. (2008) reported a phosphorus removal of only 5% during anaerobic treatment of black water collected with conventional flush toilets (6L per flush), i.e. 95% was conserved in the effluent. However, for more concentrated black water, collected with vacuum toilets (1L per flush), 40% of the phosphorus was removed when applying UASB-septic tank (Kujawa-Roeleveld et al., 2005).

Prospects of Source-Separation-Based Sanitation Concepts: A Model-Based Study

“…By replacing the urine transport with a microbial fuel cell, the total primary energy consumption can be decreased by 19% in Concept 2 and 17%-23% in Concept 4, indicating a promising new direction for urine treatment. According to the current Dutch guidelines for sewage sludge reuse in agriculture (BOOM), reuse of black water sludge is prohibited, due to elevated concentrations of copper and zinc [41]. However, as black water is predominantly human originated (urine, feces and tap water), the applicability of sewage sludge reuse guidelines on the reuse of black water sludge can be argued.…”

“…where m N aOH is the mass of NaOH (g/L), M is the molecular mass (g/mol), pH a is the influent pH of 7.7 [33] and pH b is the operational pH of 9 [35]. Consumption of 33% NaOH was further determined from the mass of NaOH.…”

“…The storage time was 1 d for the UASB buffer tank (assumed), 0.3 d for the SBR, A-trap and MBR buffer tanks (assumed) and six months for the urine collection tank [7]. The HRT was 15 d for the digester [22], 0.08 d for the struvite reactor [35], 1.9 h for the MBR [12], 4 min and 54 min for the aerated grit chamber and settling tank, respectively [57], and 1.9 h for the A-trap reactor [34]. The HRT of the UASB reactor was calculated according to Zeeman and Lettinga [56] [Equation (A8)]:…”

Recovery of nutrients from biogas digestate with biochar and clinoptilolite

“…In this pH range struvite (MgNH4PO4.6H2O) precipitation may also be responsible for ammonium removal (De Graaff et al, 2011). However no further measurements were conducted to explain which mechanism was dominant for ammonium removal.…”

“…The very different removal from synthetic solution and the liquid fraction of digestate for the H2SO4 treatment may be attributed to the difference in pH between the two solutions. All liquid digestates had a final pH (at the end of the sorption experiment) of between 7.42 and 8.87 (Table 3.3), which would be within a suitable pH range for hydroxyapatite and struvite precipitation (De Graaff et al, 2011). Moreover, cation bridging of divalent and trivalent cations (which were not likely to be present in acid-treated biochar, but would be present in the digestate) with orthophosphate may explain the differences in orthophosphate removal between the synthetic solution and the liquid fraction of digestate (Lin et al, 2012;Qian et al, 2013).…”