design and economical parameters

A WWTP of about 300 megaliter per day (MLD) with an air emission of 50000 m³ h⁻¹ with 39 model VOC odorants, H2S and methylmercaptan at trace level concentrations and 40% of relative humidity was selected as a model malodorous emission for the economical and energy efficiency analysis (Zarra et  al. 2008;

Barbosa et al. 2002; Estrada et al. 2011). All technologies evaluated were designed to support removal efficiencies >99% for H2S and >95% for odour concentration, except for activated sludge diffusion and activated sludge recycling, where 75%

and 50% odour removal efficiencies were considered as realistic values according to their current development state or, in the case of the ASD, due to the possibility of the technology not being able to treat all the odorous emission produced in the plant. Typical design and operational parameters of these technologies are summarized in Table 9.1.

All investment and operating costs of the most commonly used odour abatement technologies were based on Estrada et al. (2011) and (2012) and updated according to the most recent available Chemical Engineering Process Cost Index (CEPCI) 2012 and the UBS Prices and Earnings 2012 report. Table 9.2 presents a summary of the typical costs applied in the analysis.

Chemical scrubbing (CS). A two-stage NaOH-NaClO process with a total height of 2 m, packed with Intalox Saddles and operated at an empty bed residence time (EBRT) of 4 s (2 s per stage), was considered in this study. Packing material lifespan, purchase costs and disposal costs were set up at 10 years, 1370 EUR m⁻³ and 137 EUR m⁻³, respectively. The total pressure drop in the system, including ductwork, was estimated at 1000 Pa, and water was recycled at a rate of 180 L m⁻³ min⁻¹. Labour costs of 20700 EUR per media substitution were considered.

Odour abatement technologies in WWTPs 169

table 9.1 Data compilation for the design parameters and operating costs for the different technologies evaluated. technologyheightpacking materialEBrtpacking material lifespan packing material purchase costs packing material disposal costs labour, transport and handling costs

pressure dropothers chemical scrubbing (cS) two-stage NaOH-NaClO

2 mIntalox Saddles4 s (2 s per stage) 10 years1370 EUR m−3137 EUR m−320700 EUR per media substitution

1000 PaLiquid recirculation: 180 L m−3 min−1 Activated carbon filtration (Ac)

0.6 mGranular impregnated AC (450 kg m−3)

2.5 s6 months5.5 EUR kg−1137 EUR m−320700 EUR year−12250 PaNo regeneration of the activated carbon was considered in the estimation of lifespan and costs Biofiltration (BF)1 mCompost60 s2 years82 EUR m−348 EUR m−333130 EUR year−11500 PaHumidification: 0.02 kg water (kg air)−1 Irrigation by means of 2 water nets with 49 drips m−2 and each drip irrigating 1.9 L h−1 for 3 min day−1 Biotrickling Filtration (BtF)

4 m (2 m per stage)Inert polyurethane foam (PUF)15 s10 years1370 EUR m−3137 EUR m−320700 EUR per media substitution 1000 PaLiquid recirculation: 7.2 L m−3 min−1. Liquid renewal rate: 2.5 L per g of H2S removed (Continued)

table 9.1 Data compilation for the design parameters and operating costs for the different technologies evaluated (Continued). technologyheightpacking materialEBrtpacking material lifespan packing Material purchase costs packing Material disposal costs labour, transport and handling costs

pressure dropothers 1 stage BtF+ Ac2 m+ 0.6 mPUF + Standard AC9 s + 2.5 sPUF: 10 years. AC: 2 years

1370 EUR m−3 for PUF. 4.1 EUR kg−1 for AC

137 EUR m−322770 EUR year−12500 PaExtended lifespan of 2 years for AC due to the lower concentration of odorants to be treated in the adsorption unit. 1 stage cS+ Ac2 m+ 0.6 mIntalox Saddles + Standard AC2 s + 2.5 sIntalox Saddles: 10 years. AC: 2 years

1370 EUR m−3 for Intalox Saddles. 4.1 EUR kg−1 for AC 137 EUR m−322770 EUR year−12500 PaExtended lifespan of 2 years for AC due to the lower concentration of odorants to be treated in the adsorption unit. 1 stage BtF+ 1 stage cS

2 m+ 2 mPUF + Intalox Saddles9 s + 2 s10 years1370 EUR m−3137 EUR m−320700 EUR per media substitution

1500 Pa Step Feed BF1 mCompost60 s2.5 years82 EUR m−348 EUR m−324850 EUR year−11250 PaBF modified by supplying the odorous emission in three different locations along the BF height (Estradaet al. 2013b).

Odour abatement technologies in WWTPs 171

Activated carbon filtration (AC). The adsorbent selected for the filtration process was granular impregnated activated carbon, characterized by a density of 450 kg m⁻³, purchase cost of 5.5 EUR kg⁻¹ and a lifespan of 6 months (no regeneration of the activated carbon was considered). The adsorption filter consisted of a 0.6 m height column operated at an EBRT of 2.5 s, resulting in a total pressure drop of 2250 Pa including grease filters and ductwork. The disposal costs of activated carbon were 137 EUR m⁻³, while AC transport and renewal costs added up to 20700 EUR year⁻¹.

Biofiltration (BF). A 1 m biofilter packed with compost with a lifespan of 2 years was selected in this study. The system operated at an EBRT of 60 s and a total pressure drop of 1500 Pa (including the pressure drop of the humidifier and ductwork). Irrigation of the biofilter was performed by means of 2 water nets located at the top of the unit, each of them provided with 49 drips m⁻² and each drip irrigating 1.9 L h⁻¹ for 3 min day⁻¹. Total humidification requirements were estimated to be 0.02 kg water (kg air)⁻¹. The packing purchase costs were 82 EUR m⁻³, while the costs associated to its disposal and transport-handling were 48 EUR m⁻³ and 33130 EUR year⁻¹, respectively.

Biotrickling filtration (BTF). A two-stage BTF operated at acid (~2) and neutral pH, respectively, with a total height of about 4 m (2 m per stage) was considered as model BTF. Inert PUF, with a lifespan of 10 years and a cost of 1370 EUR m⁻³, was selected as the packing material. Disposal and labour costs were estimated at 137 EUR m⁻³ and 20700 EUR per media substitution, respectively. The total pressure drop in the system and ductwork was 1000 Pa. The EBRT, liquid recycling and liquid renewal rate were set at 15 s, 7.2 L m⁻³ min⁻¹ and 2.5 L (gH2S removed)⁻¹, respectively.

BTF + AC. This hybrid technology consisted of a single stage BTF and an AC

filter acting as a polishing step. Similar operating parameters as in stand-alone technologies were used in the hybrid technology except for a shorter EBRT of 9 s in the BTF and the use of standard activated carbon with a price of 4.1 EUR kg⁻¹ and an extended lifespan of the packing material of up to 2 years due to the lower table 9.2 Data compilation for the operating costs based on previous studies by Estrada et al. 2012 (updated to 2012).

Source value remarks

Energy EUR 0.105 kW⁻¹

Chemicals Caustic soda EUR 0.175 kg⁻¹ 50% w/w, density 1.53 kg l⁻¹ Hypochlorite EUR 0.210 kg⁻¹ 12.5% w/w, density 1.22 kg l⁻¹ Activated

concentration of odorants to be treated in the unit after the treatment in the BTF.

The total pressure drop of this combined system was 2500 Pa.

CS + AC. A single-stage CS operated at an EBRT of 2 s was coupled with an

AC as a polishing step. The rest of the operating parameters were maintained as in the stand-alone technologies except for the use of standard activated carbon packing with a price of 4.1 EUR kg⁻¹ and extended lifespan of 2 years due to the lower concentration of odorants to be treated in the unit. A total pressure drop of 2500 Pa was considered.

BTF + CS. A single-stage CS acts as the polishing stage after a single-stage BTF in this hybrid technology. EBRTs in the BTF and the CS stages can be set at 9 s and 2 s, respectively, to fulfil the target odour and H2S REs. The rest of the operating parameters were maintained as in the stand-alone technologies. The total pressure drop of this combined system was 1500 Pa.

Step-feed biofilter (Step BF). The operation of the standard BF was modified by supplying the odourous emission in three different locations along the BF height (Estrada et al. 2013b). This configuration allows for an increased packing lifespan of 25% compared to the conventional BF, while reducing the overall pressure drop of the bed by 25% (total pressure drop in the step-feed BF of 1250 Pa).

Activated sludge diffusion (ASD). The odorous emission is sparged into an activated sludge tank (devoted to wastewater treatment) with a depth of 4 m. An additional pressure drop of 500 Pa was considered to take into account the ducting required to conduct the emission to the aeration basin. Grease filters, corrosion resistant blower, upgrade to fine bubble diffusers and instrumentation were included as capital costs.

Activated sludge recycling (ASR). A sludge flowrate of 625 m³ h⁻¹ is pumped from the secondary settler of the activated sludge tank to the head of the WWTP, representing 5% of the total wastewater flowrate treated in the plant (a model WWTP treating 300 megaliter per day was considered, typical size of a WWTP with approximately 50000 m³ h⁻¹ malodorous air emission). Piping, sludge pumps, dispensers, valves, instrumentation and automation needed were included in the costs of this technology.

The CO2 footprint for each technology was calculated according to the data shown in Table 9.3. The following transportation distances were assigned to the different materials required in the technologies evaluated: 50 km of road transportation to the compost needed in BF based on the possibility of locally purchasing this packing material; 500 km of road transportation to the polyurethane foam (PUF) and Intalox Saddles required in the BTF and CS, respectively, based on the possibility of purchasing these materials inside the country; 5000 km of sea transportation + 200 km of road transportation to the activated carbon, according the present trend of activated carbon purchase from Asian manufacturers. Finally, 15 km of road transportation were considered for the disposal of all spent packing materials in local landfills.

Odour abatement technologies in WWTPs 173

table 9.3 Data compilation for the calculation of CO₂ footprint.

Source co₂

Compost 600 Boldrin et al. (2009)

Activated carbon

1000 Agentschap (2012)

Transport Road (Truck) 0.127 kg CO2-eq ttransported−1

km⁻¹

IMO (2009)

Sea (Ship) 0.015 IMO (2009)

9.3 coMpArAtIvE pArAMEtrIc EFFIcIEncy

AnAlySIS