Impact of new sanitation technologies upon conventional wastewater infrastructures

Abstract

Separation of wastewater split streams and utilization of the resources contained therein are an important prerequisite for a sustainable wastewater treatment. In areas with existing infrastructures, this leads to transition states during which current systems must continue to operate. A simple mass-flow based method was developed to assess the impacts of an incremental transition (co-digestion of blackwater and sewage sludge) upon plant operation. The results proved that blackwater co-digestion could be easily integrated into existing wastewater infrastructures; however, beyond 8% transition thickening was required due to hydraulic limitations in the digester. Additionally, nitrogen recovery was necessary beyond 35% transition to avoid unfavorable C:N ratios. The investigated concept also led to a power gain of 11 kWh/(PE∙a) due to enhanced biogas production and aeration savings. The developed Excel-based method was considered suitable for evaluating the benefits and probable tipping points for plant operation during transition to source-separated sanitation systems.

Keywords:

• Blackwater digestion
• nutrient recovery
• resource-oriented sanitation systems

Introduction

Transition to sustainable urban systems is only possible through integrating and closing water, energy and material flows (van der Hoek et al. 2015). Slagstad and Brattebø (2013) found out that among water and wastewater infrastructures wastewater treatment plants (WWTPs) contributed most significantly to the total environmental impact mostly due to high requirements of energy and chemicals. For instance, conventional phosphorous removal often uses costly chemicals such as ferric chloride, alum and polymers (Tchobanoglous et al. 2014). Indeed, in many municipalities wastewater facilities are the largest electricity consumers (Nowak et al. 2015) mostly because of energy consumption for pumping and aeration (Muga and Mihelcic 2008) - particularly the conventional nitrogen removal is energy-intensive (Lackner et al. 2014). Although in some countries many efforts have been undertaken to improve the energy balance of WWTPs, these usually do not directly address the recovery of resources in municipal wastewater (energy, nutrients and water) as wastewater is typically regarded as merely being an energy sink.

Nutrient recovery is not state-of-the-art at WWTPs, yet it has been tested in both lab and pilot-scale. Some authors (e.g. Wang and Peng 2010, Am Yusof et al. 2010) have suggested application of zeolites in the wastewater treatment because of their capability to adsorb ammonium, while being abundant and inexpensive (the ion-exchange properties of zeolites are well described in the scientific literature, e.g. Wang and Peng 2010). Other authors have proposed nutrient recovery through microalgae cultivation technologies (e.g. Cai et al. 2013); these are capable of incorporating nitrogen and phosphorus (required for cell growth) from wastewater under controlled environments. However, the high flow rates and low nutrient concentrations of conventional wastewater has hindered a large-scale implementation of these technologies. Phosphorus can be recovered through chemical precipitation from WWTPs as struvite or calcium phosphate (Driver et al. 2010) but current approaches for phosphorus recovery from sludge liquor only bring about low recovery rates (e.g. Cornel and Schaum 2009) or involve the use of costly chemicals to redissolve metal phosphates by acidic leaching of digested sludge (cf. Meyer et al. 2015). In view of all that, separate collection and selective treatment of blackwater would likely improve the applicability of current techniques and help overcome technical difficulties.

Resource-oriented systems prioritize water reuse, energy recovery and recycling of wastewater constituents, e.g. phosphorous, nitrogen, organic substances, through separate collection and selective treatment of wastewater split streams (WSWU and DWA 2016). Apart from relatively low flow rates, it was shown that blackwater accounts for 67% BOD₅, 52% COD, 92% N_tot and 75% P_tot in domestic wastewater (DWA 2014b). Thus, blackwater is more suitable for anaerobic treatment (biogas production and energy use) and nutrient recovery than wastewater.

Many authors have suggested on-site or semi-centralized blackwater digestion (e.g. Wendland et al. 2007, de Graaff et al. 2010, Zeeman and Kujawa-Roeleveld 2013). With respect to treatment, Wendland et al. (2007) reported a COD removal of 61% in an anaerobic lab-scale continuously stirred tank reactor treating concentrated blackwater (T = 37 °C; HRT = 20 d). This implies that blackwater digestion is a suitable treatment technology but alone not sufficient to comply with legal requirements for the wastewater discharge.

With regard to greywater, decentralized treatment and recycling have been successfully implemented in several rural areas worldwide. Pidou et al. (2007) analyzed numerous studies In addition, Penn et al. (2012) reported 26–41% reduction of the daily household wastewater flows by reuse of light greywater. Recycling of treated greywater for toilet flushing was shown to be an efficient method for minimization of drinking water consumption in a densely populated semi-urban area (cf. Zhang et al. 2009).

Within some pilot projects, the infrastructure to collect blackwater at low dilution has been implemented over the last decade (Nowak et al. 2015) but mostly within new settlements (decentralized systems). Indeed, processes and ideas for source-separated sanitation systems have already been thoroughly discussed (e.g. Kaufmann Alves 2013, Kjerstadius et al. 2015); however, centralized transition concepts, probable transition pathways and impacts upon plant operation during transition have thus far not been evaluated in any study.

Particularly in countries where centralized wastewater infrastructures with long service lives are present, an incremental shift from conventional to resource-oriented systems will have to be undertaken eventually, for instance through the treatment of concentrated blackwater in anaerobic digesters to improve biogas production and nutrient recovery. In order to avoid treatment failure the operation of the centralized WWTP must be investigated and assessed during different transition states.

This study proposed the direct conveyance of blackwater through vacuum sewers to municipal digesters, based on the knowledge that in the EU anaerobic sludge digestion is used in 24 of the 27 member states (Kelessidis and Stasinakis 2012) and that many digesters are overdimensioned, so hydraulic capacities for co-digestion exist (Dichtl and Schmelz 2015). To the best of the authors’ knowledge, no one has so far considered a transition concept in which blackwater is co-digested in municipal digesters alongside primary and excess sludge. Critical transition states in which operating problems may arise were assessed and discussed. Additionally, changes in energy demand, biogas production and nutrient reuse were quantified for the transition concepts proposed. For that a simplified mass-flow based algorithm has been developed, so that comparisons among different transition scenarios could be drawn and critical tipping points assessed, after which conventional systems would not operate satisfactorily without altering process conditions significantly.

Due to possible operating bottlenecks during transition to source-separated sanitation such as wastewater stagnation, hydraulic overloading in the anaerobic stage, C:N shift in the anoxic stage or even underload-related problems in the aerobic stage, etc., which can be further aggravated by decoupling greywater, a very thorough analysis has been undertaken within this study. Also, corrective actions were proposed in order to assure a safe and stable plant operation as well as good cleaning efficiencies (the main objective of a wastewater treatment facility).

Materials and methods

Within this study impacts of an incremental blackwater co-digestion and greywater recycling upon the functionality of a conventional wastewater treatment plant were investigated using mass and volume balances for different transition states. “Transition” was defined as the fraction of inhabitants within a catchment area using vacuum toilets for direct blackwater discharge to municipal digesters of WWTPs whereas for greywater two cases were investigated:

(1)	Residual wastewater transport via the existing combined sewer to the aerobic stage of the WWTP (case one).
(2)	Greywater decoupling from the plant and decentralized treatment (case two).

Special focus was given to the transition state immediately after which operating problems would occur, mainly in denitrification or anaerobic digestion. The compliance with wastewater surveillance values was of highest priority. Furthermore, benefits concerning energy balance and nutrient recovery were investigated.

Sources and assumptions used for the mass and volume flow balances are listed in Table 1. The inhabitant-specific balances (applicable to all WWTPs with anaerobic sludge stabilization) are mainly founded on typical data from literature (cf. Table 1), yet the lack of representative data with regard to the transition to source-separated sanitation was the most challenging problem within this study due to insufficient experience in the research field. These difficulties could be overcome by using own data (where applicable) and simplifying a few calculation processes (cf. Table 1). For clarity a value of 100,000 PE was set as a reference (model WWTP). The loads used are 85 percentile values. In the model WWTP a nitrification and an upstream denitrification stage as well as a digester for separate sewage sludge stabilization with available hydraulic reserves (HRT_actual = 25 d; HRT_desired = 20 d) were assumed.

Table 1. Assumptions, calculations and references for mass and volume balances in the actual state if not otherwise stated.

Wastewater stream or process step	Loads, volume flows, TS contents and further parameters
Raw wastewater (WWTP influent)	BOD5 = 60; COD = 120; Ntot = 11; Ptot = 1.8 g/(PE∙d) (ATV-DVWK 2000a) Dry weather flow Q = 175 L/(PE∙d) (DWA 2014a): ca. 120 L/(PE∙d) household WW+55 L/(PE∙d) infiltration water
Primary sludge	Q = 1 L/(PE∙d) ^a ; TSS = 4% ^b (DWA 2014a) BOD5 = 20; COD = 40; Ntot = 1; Ptot = 0.2 g/(PE∙d)^1 (ATV-DVWK, 2000a)
Excess sludge	Q = 5.1 L/(PE∙d)^1, ^c ; VS = 25.1 g/(PE∙d)^1,3; TSS = 0.7% ^d (DWA 2014a) With COD/VS = 1.5; Ntot/VS = 0.1 in g/g (ATV-DVWK 2000b). COD = 38; Ntot = 2.5 g/(PE∙d); P = dependent on P elimination extent
Treated wastewater (WWTP effluent)	Compliance with 50% of the emissions standards stated in AbwV. (2004) for WWTp>6,000 kg BOD5/d COD = 37.5; Ntot = 6.5; Ptot = 0.5 mg/l ^e
Thickened sludge	TSS = 4%; Q = 1.9 L/(PE·d) with TSSthin = 1.3%
Sludge liquor (after thickening)	Own investigations: dissolved fractions in mixed sludge: CODmf/COD = 0.09; NH4-N/Ntot = 0.15 in g/g. Ptot neglected (chemical P elimination assumed). COD = 5 g/(PE·d); Ntot = 0.4 g/(PE·d).
Sludge liquor (after dewatering)	Filter press; sludge conditioned with lime and iron salts (Dichtl and Schmelz, 2015) COD/BOD5 = 2.6; COD = 2350 mg/l ^f Ntot≈1.5 g/(PE∙d) ^g ; Ptot≈5% of 1.8 g/(PE∙d) = 0.09 g/(PE∙d) (ATV-DVWK 2000b)
Activated sludge process	Estimated after mass balance: 45 g COD/(PE∙d); NNitrification = (8.2+1.1) g N/(PE∙d) with 4.33 g O2/g N; NDenitrification = 8.2 g N/(PE∙d) with 2.86 g O2/g N. O2 concentration in aeration tank = 2 mg/L;O2 saturation of 10 mg/L; oxygen mass transfer coefficient ratio α = 0.6. Assumption of ηaeration: 2 kg O2/kWh. Oxygen demand in pure water: ODC = 45 g O2/(PE∙d); ODN = 40 g O2/(PE∙d); ODDN = 23 g O2/(PE∙d), carbon savings Oxygen demand in sludge water: OD = 128 g O2/(PE∙d); Power demand for aeration = 23 kWh/(PE∙a), cf. (Steinmetz et al. 2015) ^h
Digester Digested sludge	COD elimination = 50%; TSSDS = 2.8% ^i (Dichtl and Schmelz, 2015) Calculation of energy demand for sludge heating (T = 10–15 °C); no heat exchanger considered; substrate thermal capacity≈4.2 kJ/kg; Q = 1.9 L/(PE·d) for thickened sewage sludge. Tdigestion = 37 °C Thermal energy demand = 20 kWh/(PE∙d)
Digested sludge (after dewatering)	TSSDS,dewatered = 28% ^j (Dichtl and Schmelz 2015)
Digester gas	Energy generation estimated after COD balance (36.5 g COD/(PE∙d) in biogas) with: 2.86 kg COD/LMethane at STP (DWA 2015); calorific value of 10 kWh/m^3 in CH4 at STP (DWA 2014a); ηCHP,electrical = 35% Electricity self-supply = 16 kWh/(PE·a); Heat self-supply = 30 kWh/(PE·a)
Blackwater ^k	Own investigations and literature, cf. and (DWA 2014a). CODBW = 0.6 CODWW; Ntot,BW = 0.8 Ntot,WW; Ptot,BW = 0.9 Ptot,WW; TSS = 0.9%; Q = 7 L/(PE·d) ^l ; COD = 72 g/(PE·d); Ntot = 8.8 g/(PE·d); Ptot = 1.62 g/(PE·d)
Blackwater, thickened ^k,m	Own investigations: TSS = 4%; Q = 0.9/4·7 = 1.6 L/(PE·d)
Blackwater supernatant ^k,m	Own investigations and literature for dissolved fractions in blackwater (de Graaff et al. 2010, Wendland et al. 2007, Kujawa-Roeleveld et al. 2006): CODmf/COD = 0.3; NH4-N/Ntot = 0.75; Ptot,mf/Ptot = 0.35 in g/g. COD = 17 g/(PE·d); Ntot = 5.1 g/(PE·d); Ptot = 0.4 g/(PE·d)
Greywater ^k	Calculated after 121 L/(PE·d) water consumption (UBA 2015) and average usage of 33 L/(PE·d) flush water (BDEW 2011) Q = 121–33 = 88 L/(PE·d)

^a with 2.0 h flow time in primary clarifier.

^b typical range: TSS_PS 3–6%.

^c with T = 15 °C, 15 d sludge retention time and T_design = 12 °C in activated sludge process.

^d typical range: TSS_ES = 0.6–0.8%.

^e effluent concentrations were assumed 50% of permissible emission standards in Germany.

^f typical ranges: BOD₅ = 300–1,500 mg/l; COD = 1200–3500 mg/l.

^g large-scale investigation for n = 204 WWTP in Germany with anaerobic mesophilic stabilization.

^h 51.9 kWh/(PE·d) electricity demand (85 percentile value for WWTPs > 10,000 PE; n = 62); aeration in the biological stage ≈19 kWh/(PE·d).

ⁱ typical range TSS_DS = 0.9–5.9%, large-scale investigations.

^j typical range TSS_DS,dewatered = 20–37%, large-scale investigations.

^k 100% segregated sanitation. Separate collection of blackwater (vacuum toilets) and greywater.

^l 1.37 L/(PE·d) urine + 0.14 L/(PE·d) feces (WSWU, and DWA 2016)+ 5.5 flushes/PE·1 L/flush = 7.0 L/(PE·d).

^m after thickening from 0.9 to 4.0% TSS.

The mass-flow method proposed is an Excel-based algorithm which comprises iterative calculations coupled either with load or flow rate values (for further information, cf. Table 1). For instance, loads and volumes of separated blackwater and greywater vary with increasing transition, which leads to changes in volume and composition of further streams such as WWTP influent, excess and primary sludge, influent to the digester etc. Infiltration water was assumed to be constant. The chargeback of dewatering and thickening stages was considered into the calculations, as the chargeback tends to increase with increasing transition.

Figure 1 depicts the actual state of the model WWTP with specific COD, N_tot and P_tot loads and flow rates, thus allowing a more comprehensive understanding of the relationships among the different parameters stated in Table 1. The total solids contents were occasionally indicated to clarify the volume flow distribution after thickening or dewatering and assumed constant for several streams such as blackwater, primary and excess sludge, so flow rates varied accordingly within the transition scenarios. It is important to note that the bars merely provide a qualitative understanding (two scales depicted), whereas the absolute values allow any quantitative analysis.

Figure 1. Mass and volume balances for the actual state (0% transition) of a conventional WWTP, with: PC = primary clarifier; DN = denitrification stage; SC = secondary clarifier; ST = sewage sludge thickener; DGT = digester; FP = chamber filter press (for sewage sludge dewatering); PS = primary sludge; ES = excess sludge; RS = raw sludge; DS = digested sludge; SL = sludge liquor (chargeback).

Results

Process stability

Based on the mass balances it could be shown for case one (blackwater co-digestion and treatment of residual wastewater including greywater in the activated sludge tank) that the existing infrastructure is capable of operating normally up to approx. 8% transition (i.e. for the model WWTP up to a blackwater co-digestion of 8000 PE); after this point the digester’s hydraulic reserves are exhausted. This effect was ascribed to the higher volume flows of blackwater – 7 L/(PE∙d) at 0.9% TSS – compared to pre-thickened sewage sludge – approx. 1.9 L/(PE∙d) at 4% TSS. As a result, a few structural alterations of the WWTP must be undertaken from 8% transition onwards. A static or mechanical thickener, e.g. settling tank, emerges as an alternative, as preliminary lab-investigations showed that blackwater has better settleability than conventional excess sludge.

Attaining an unfavorable BOD₅:N_tot ratio in the aerobic stage may be a critical point for plants undergoing transition because of the displacement of BOD₅ to the anaerobic stage. This proved critical at 35% blackwater separation (for the model WWTP 35,000 PE). BOD₅:N_tot ratios < 3.5 would be expected hereafter, as can be inferred from Figure 2. An incomplete denitrification would rapidly reflect in increasing nitrate effluent values, which has to be avoided. Denitrification problems can be counteracted by sidestream N recovery processes (processes implemented on N-rich sidestreams within sewage sludge processing lines), as these emerge as an alternative to conventional N removal and contribute to resource-efficiency in wastewater treatment. Minimum nitrogen recovery rates of 20% (in terms of N in both sludge liquor and blackwater supernatant after thickening) have proved to be sufficient to not compromise the functionality of the nitrogen removal stage in the WWTP up to 100% transition.

Figure 2. Mass and volume balances for 35% blackwater separation (e.g. 35,000 PE out of 100,000 PE) of a conventional WWTP undergoing transition to new sanitation technologies, with: BW = blackwater; BT = blackwater thickener, cf. Figure 1 for remaining legends.

It can be inferred from Figure 3 that blackwater thickening enables additional blackwater loading up to approx. 75,000 PE (at 4% TSS). Assuming 5% TSS in thickened sludge volume balances proved blackwater co-digestion possible up to 100% blackwater separation.

Figure 3. Mass and volume balances for 75% blackwater separation (e.g. 75,000 PE out of 100,000 PE) of a conventional WWTP undergoing transition to new sanitation technologies, with: BW = blackwater; BT = blackwater thickener; N = nitrogen recovery, cf. Figure 1for remaining legends.

In case two, blackwater was treated in the digester (like case one), yet decentralized treatment and recycling of greywater was considered. Thus, less BOD₅ reached the WWTP influent, deteriorating denitrification at an earlier transition state (15,000 rather than 35,000 PE). This required, at 75% transition, a minimum N recovery rate of 50% in sludge liquor and blackwater supernatant for maintenance of a favorable BOD₅:N_tot ratio for denitrification, which is higher than in case one.

A summary of the tipping points for plant operation and respective corrective measures to overcome these problems is given in Table 2.

Table 2. Transitions states with respective tipping points for plant operation and corrective measures to overcome operating problems.

Transition scenario	Tipping points and impacts upon plant operation	Measures to overcome operating problems
8% transition: vacuum toilets to separate BW and GW of 8% of the inhabitants, BW treatment in the municipal digester, GW treatment in the municipal activated sludge process	• Hydraulic overloading in the digester leads to HRT < 20 d (insufficient under mesophilic conditions)	• BW thickening (to 4% TSS)
35% transition: vacuum toilets to separate BW and GW of 35% of the inhabitants, BW treatment in the municipal digester, GW treatment in the municipal activated sludge process	• Increase in N effluent values due to insufficient denitrification (BOD/N < 3.5:1)	• N recovery from sludge and blackwater supernatant (preferred alternative for sustainability reasons) • Sidestream N removal • External carbon source for denitrification
75% transition: vacuum toilets to separate BW and GW of 75% of the inhabitants, BW treatment in the municipal digester, GW treatment in the municipal activated sludge process	• Hydraulic overloading in the digester leads to HRT < 20 d (insufficient under mesophilic conditions) • Increase in N effluent values due to insufficient denitrification (BOD/N < 3.5:1)	• Further thickening of blackwater to 5% TSS, e.g. through mechanical thickening • Enhanced thickening of PS/ES • Nitrogen recovery from sludge and blackwater supernatant (minimum of 20% recovery efficiency) • Sidestream N-removal • External carbon source for denitrification
9% transition: vacuum toilets to separate BW and GW of 9% of the inhabitants, BW treatment in the municipal digester, GW treatment on-site (no treatment in existing WWTP)	• Hydraulic overloading in the digester leads to HRT < 20 d (insufficient under mesophilic conditions)	• BW thickening (to 4% TSS)
15% transition: vacuum toilets to separate BW and GW of 15% of the inhabitants, BW treatment in the municipal digester, GW treatment on-site (no treatment in existing WWTP)	• Increase in N effluent values due to insufficient denitrification (BOD/N < 3.5 : 1)	• N recovery from sludge and blackwater supernatant • Sid stream N removal • External carbon source for denitrification
75% transition: vacuum toilets to separate BW and GW of 75% of the inhabitants, BW treatment in the municipal digester, GW treatment on-site (no treatment in existing WWTP)	• Increase in N effluent values due to insufficient denitrification (BOD/N < 3.5 : 1)	• Nitrogen recovery from sludge and blackwater supernatant (minimum of 50% recovery efficiency) • Sidestream N-removal • External carbon source for denitrification

Note:

BW = blackwater; GW = greywater; HRT = hydraulic retention time; WWTP = wastewater treatment plant.

Energy demand and energy production

Aeration in the aerobic stage is the principal power consumer in WWTPs. By displacing carbon and nitrogen (contained in blackwater) to the anaerobic stage, aeration requirements could be greatly reduced, as depicted in Figure 4 (for both case one and two).

Figure 4. Expected developments of power demand for aeration and power generation from biogas in a conventional WWTP for cases one and two.

For case one, it was observed that the energy demand for aeration could be reduced in total by approx. 7.5 kWh/(PE·a) at 75% transition, which corresponded to 30% of the actual state. It can be inferred from Figure 4 that a slightly heightened power demand for aeration of approx. 0.5 kWh/(PE·a) was observed immediately after the set-up of blackwater thickening at 8% transition due to recirculation of the blackwater thickening supernatant. This had only a marginal impact on biogas production. Furthermore, with the implementation of a nitrogen recovery process (η = 50%) from both sludge liquor and blackwater supernatant at 35% transition, the power required for aeration fell abruptly from approx. 21 to 20 kWh/(PE·a), as can be seen in Figure 4. In parallel the power generation could be increased to approx. 3.5 kWh/(PE·a) in total, which is a 20% increase on the actual state, so a total energy gain of 11 kWh/(PE·a) could be achieved. All in all, WWTPs may become energy producers rather than consumers, while greatly decreasing their environmental impact.

For case two, Figure 4 shows that under energy aspects decoupling of greywater from the WWTP provided no benefits other than those observed in case one. While in the course of transition energy requirements for aeration sank relatively rapidly from 23 to 10 kWh/(PE·a) due to both nitrogen recovery and further load decoupling from the activated sludge process, no increased biogas production was reported (in view of reduced amounts of primary sludge and excess sludge) but rather 10% reduction in power generation at 75% transition, in contrast to case one. Nevertheless, within an overall wastewater management scheme the decentralized treatment and usage of greywater might have further advantages.

Additionally, it was shown for case one that heat production from biogas increased from 30 to 36 kWh/(PE·a) at 75% transition; yet, the energy required for sludge heating also increased from 20 to 25 kWh/(PE·a) due to higher substrate volumes at the digester’s inlet, so no net thermal energy gain can be expected.

Nutrient recovery

Nitrogen recovery

In order to further comply with nitrogen regulation standards from 35% transition onwards, nitrogen recovery processes from N-rich sidestreams (from both sludge liquor and blackwater supernatant after set-up of blackwater thickening) were considered. Figure 5 shows that the total N recovery potential increases with increasing transition. At 35% transition, a critical point for the WWTP operation, a nitrogen removal efficiency of ƞ = 60% (in terms of N in sludge liquor and blackwater supernatant) brought about a recovery rate of approx. 20% (in terms of N load to the WWTP) or correspondingly 2.2 g NH₄-N/(PE·d). An efficiency of 30% yielded 1.1 g NH₄-N/(PE·d). The results show that nitrogen can be partially recovered during transition to resource-oriented sanitation systems, which implies that mineral fertilizers can be partially substituted with recycling products from WWTPs. Additionally, the energy demand and costs for aeration can be significantly decreased.

Figure 5. Total nitrogen recovery potential and recovery rates ¹ at two different recovery efficiencies.

Phosphorus recovery

In contrast to nitrogen, phosphorus recovery is not essential for the functionality of the WWTP in the long-term, because it can be eliminated for instance by chemical processes. Nevertheless, phosphorus is a finite resource and other ways rather than unsustainable extraction from rock phosphate must be considered in the mid-term.

Figure 6 depicts phosphorus potentials in both digested sludge and blackwater supernatant (after blackwater thickening set-up). As P accumulates in digested sludge, theoretical P recovery potentials in digested sludge are very high; however, recovery rates are in fact proportional to the extent of P release e.g. by acidic leaching of digested sludge. A total recovery efficiency of 40–60% (in terms of P in digested substrate and blackwater supernatant) was assumed. This is paralleled by a P recovery rate of 38–57% (about 0.7–1 g PO₄-P/(PE·d)). All in all, relatively high phosphorus recovery potentials were reported, if the P fraction contained in the blackwater supernatant is considered.

Figure 6. Total phosphorus recovery potential and recovery rates ² at two different recovery efficiencies.

With respect to nutrient utilization, greywater decoupling from the plant (case two) reported just slightly lower P and N recovery rates, as greywater retains only a minor fraction of the N and P found in domestic wastewater (cf. DWA 2014b). Therefore, greywater decoupling is not discussed in this section.

The results of the algorithm proposed corroborate the applicability of different transition concepts, in which blackwater is co-digested alongside primary and excess sludge in centralized WWTPs. These emphasize that the centralized WWTP can be easily integrated into different transitions concepts, while benefits with regard to energy efficiency and nutrient recovery can be achieved. It was shown that a simple Excel-based model is suitable for evaluating the benefits and probable tipping points for plant operation during transition to resource-oriented systems.

Conclusions

It is evident that resource-oriented systems are a necessity for longer-term wastewater treatment. The mass-flow based method developed has proved to be appropriate for assessing probable impacts of existing WWTPs undergoing transition to resource-oriented systems upon plant operation. It is the first time that a simple mass-flow based method has been developed to pave sustainable transition pathways to source-separated sanitation. Under consideration of specific boundary conditions this method can be applied to any WWTP undergoing transition.

Additionally, it was shown that co-digestion of blackwater alongside primary and excess sludge is an advantageous transition concept for improving the energy balance and nutrient recovery potentials in municipal WWTPs despite the wide palette of new technical solutions, yet hydraulic reserves must be available in the digester. Also, transition is likely to be achieved with only a few process and structural alterations in the plant, e.g. set-up of nitrogen recovery and blackwater thickening. However, more comprehensive alterations in the sewer system may be required.

The findings of this study are a preliminary attempt (mainly due to the large number of assumptions made and the simplicity of the mass-flow based method reported) to assess when and where operating problems will likely occur and which benefits can be expected therefrom. A deeper analysis is only viable by consideration of real boundary conditions.

Due to the requirement of an incremental set-up of a vacuum sewer, future research should also contemplate energy balances within both the sewer system and the WWTP rather than the latter only. Additionally, transition concepts including further scenarios and technologies other than blackwater co-digestion should be assessed in future works.

Disclosure statement

No potential conflict of interest was reported by the authors.

Funding

This work was supported by the German Federal Ministry of Education and Research (BMBF) under Grant 033W011D.

Notes

1. Referred to nitrogen load to the WWTP, i.e. 11 g/(PE·d).

2. Referred to phosphorus load to the WWTP, i.e. 1.8 g/(PE·d).