Impact of new sanitation technologies upon conventional wastewater infrastructures

Figures & data

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.

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).

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.

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.

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.

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

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

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

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