Trents rural strategy Vertical flow wetlands for full wastewater - PowerPoint PPT Presentation

Bringing to life Severn Trent’s rural strategy – Vertical flow wetlands for full wastewater treatment Dr. Yadira Bajón Fernández 29 th January 2019 www.cranfield.ac.uk

Motivation - Rural strategy • Low carbon footprint (aeration / energy) • Reduced intervention • No/low sludge tankering • Societal value to local communities > 1000 sewage treatment plants ~ 750 small works (<2000 PE) • Innovation 2

Chosen technology • Proven technology (France) • Simple process that works • Lowest totex solution • 10-12 years sludge accumulation • Low / no energy • Low maintenance 3

Chosen technology 1 st stage (BOD, TSS removal) : 3 beds, 3.5 days feeding / 1week resting • 2 nd stage (Nitrification, polishing): 2 beds, 1 week feeding / 1week resting • Main layer >30cm 2-8 mm gravel 0.25 <d10 <0.40 mm sand Transition layer: 10-20cm 5-20 mm gravel 3-10 mm gravel Drainage: 10-20cm 20-40 mm gravel 20-40 mm gravel CONSENTS: 941 p.e. - TSS 50 mg/L 1 st stage 1.2 m 2 /p.e. - BOD 30 mg/L 4 2 nd stage 0.8 m 2 /p.e. - Ammonium 15 mg/L

Hulland Ward – New assets New Siphon chamber 3 x 1 st Stage vertical filters Designed @ 1.2 m 2 /PE – 0.37 m/d 3.5 d feed / 7 d rest 2 x 2 nd Stage vertical filters Pumped Designed @ 0.8 m 2 /PE 7 d feed / 7 d rest Gravity New inlet works New Storm system with Pumping to 1 st stage existing 2 Storm tanks 5

Treatment performance 1 st year of operation – 24 h composite samples Red: Expected results for two stage vertical flow reed beds based on a study of 169 full scale sites in France (Paing et al. 2015). * Based on TKN **excluding storm events (HL > 0.6 m·d -1 ) 6

Commissioning challenges Winter 2015/16 – Attempts to operate side at FFT (11 l/s) 1 st stage reduced infiltration rate lead to ponding • • Ponding caused overflows with bypass of first stage. Clog second stage 2 nd stage switched off – first stage + nSAF winter 2016/17 • • Aerated second stage Dec. 2016 7

Commissioning chellenges Reasons: Combined sewers + UK rainfall pattern VS in France. 1 st stage undersized • • Not mature beds, reeds not developed. Slow sludge mineralisation • Not critical minimium resting period Unsuitable sand on top of 2 nd stage • 8

Lessons learnt 1: Sludge layer mineralisation Time in Sludge Accumulation Dry matter OM IR operation accumulation Rate (cm) (cm/year) (%) (% of DM) mm.min- 1 December 2015- RB1 6 months 2-5 cm 8.1 76 <0.52 January 2016 RB3 3 months 2-3 cm 6.5 74 <0.39 March 2016 RB1 9 months 3-5 cm 4-7 cm 12.8 67.4 <0.35 RB3 6 months 3-5 cm (estimation) 11.5 64.5 <0.34 ≈0.6 Literature Arias 2013 1 year Site A France (winter) 0.4-0.8 cm (Molle et al. 2006) 8 years 4-7 cm 30.2 37.6 >3 year Site B France (Molle et al. 2006) (Boutin and Liénard (mixed 14 years 22.5cm (mixed sludge) >1.8 1.6 cm/year 2003) sludge)21.8 49.2 Site C France Molle et al. 2006 1 year 1 cm >21 1 cm/year Site D France Some images from August/September 2015 9

Lessons learnt 2: Hydraulic load distribution 60 Design value 50 0.37m/day 40 Insufficient reoxygenation affecting nitrification % time Retardation sludge mineralisation 30 20 10 0 0-0.2 0.2-0.4 0.4-0.6 0.6-0.8 0.8-1 1-1.2 1.2-1.4 1.4-1.6 1.6-1.8 1.8-2 >2 Hulland Ward Site A France 2011 Hydraulic load (m.d -1 ) • More than 48 h of continuous ponding • Recommended water free surface 8.5 h per day • Higher sensitivity to periodicity than intensity of hydraulic overloads • Adaption to local conditions needed 10

What do we need to achieve? 60 50 40 % time 30 20 10 0 0-0.2 0.2-0.4 0.4-0.6 0.6-0.8 0.8-1 1-1.2 1.2-1.4 1.4-1.6 1.6-1.8 1.8-2 >2 Hulland Ward Site A France 2011 1.2 m 2 ·pe Hydraulic load (m.d -1 ) Re-oxygenation of filter body -> nitrification of absorbed ammonium Sludge mineralisation -> IR 1.7 m 2 ·pe

Lessons Learnt 3: Sand specification Brix and Arias, 2005, The use of vertical flow constructed wetlands for on-site treatment of domestic 12 wastewater: New Danish guidelines. Ecological Engineering, 25: 491-500

Lessons Learnt 4: It needs time • Feeding/resting cycle impacts ammonia profile • Critical resting time 25 .Ammonia concentration in final effluent (mg/L) 20 15 10 5 0 0 50 100 150 200 250 300 350 Day operation (3 year) 9 a.m. 9 p.m. 13

Lessons Learnt 4: It needs time Sequence point* Bed 1 Bed 2 Bed 3 Beginning of the feeding period 2 nd day 1 st day 2 nd day End of the feeding period 7 th day 7 th day 7 th day Beginning of the resting day 1 st day 1 st day Middle of the resting period 7 th day 7 th day 7 th day End of the resting period 14 14 th day 14 th day 14 th day

Lessons learnt from 1 st UK system Demonstrated effective treatment. Very promising technology for future UK implementation 1. Careful control of hydraulic loading until system is mature 2. Protect minimum resting period (re-oxygenation and sludge layer mineralisation) 3. Careful sizing of first stage (Design on hydraulic profile) 4. Careful media selection 5. 2 nd stage aeration is an effective mitigation. But still working towards fully passive 6. Minimal intervention after commissioning 7. Allow 1-2 years for commissioning. We are working to accelerate maturation Would you install this scheme in other sites? Yes, but with lessons learnt 15

Current work: sludge layer Vision: control sludge layer mineralisation and characteristics (IR) by modifying operational procedures (rest/feed cycles) 16

Current work: sludge layer 1) Sludge layer properties: TS, VS, height… 17

Current work: sludge layer 2) Preferential flow pathways Cracks Voids formed by animals Voids formed by form in result of sludge Occur in result of animal (e.g. plants dewatering during resting worms) activity. Occur around stems, in period root zone and formed around plants debris/ 18

Can we accelerate crack formation?

Conclusions 1. Very promising technology for UK implementation, allowing passive or very low energy treatment with minimum intervention and deslugding 2. Careful control of hydraulic loading, specially until system is mature 3. Currently working to engineer sludge layer mineralisation in order to accelerate maturation and maintain mineralisation Would you install this scheme in other sites? Yes, but with lessons learnt 20

Any questions? Dan Cunliffe, Alex Cook, Tomas Jordan, Ricky Shepherd, Ivan Blanco, Andy Richards, Pete Vale, Richard Smith, Bruce Jefferson, Gaby Dotro, Fred Coulon, Ledicia Pereira, Antonio Labella de Tomás, Olha Khomenko