A UK water company required a 2.5× increase in biological treatment capacity at an existing WWTP serving a growing catchment, without new aeration tank construction, within a budget and programme constrained by AMP8 spending commitments.
The works was consented for 22,000 PE but was receiving flows equivalent to 38,000 PE following upstream housing development approvals. The Environment Agency had issued a Regulation 12 improvement condition requiring full compliance with a new, tighter consent (BOD 10 mg/L; SS 15 mg/L; NH&sub3;-N 5 mg/L) within 24 months. Key constraints:
Reynolds & Bauhm proposed MBBR carrier addition to the existing aeration lanes, combined with secondary clarifier hydraulic upgrades and lamella inserts:
| Stage | Equipment | Consent Performance |
|---|---|---|
| Screens & grit | Existing (retained) | TSS reduced 200 → 150 mg/L |
| Primary clarification | Existing (retained) | BOD 250 → 140 mg/L |
| MBBR / hybrid AS | K5 carriers, 55% fill; fine bubble diffusers | BOD 140 → 12 mg/L; NH&sub3; 25 → 4 mg/L |
| Secondary clarification | Lamella inserts (existing tanks) | SS 25 → 12 mg/L |
| Tertiary filters | Rapid gravity sand filters (2 no.) | SS 12 → 5 mg/L; BOD <5 mg/L |
| Disinfection | UV (existing, retained) | E. coli <1,000 cfu/100 mL |
All parameters within new consent after 6 months of operational stabilisation. Annual mean NH&sub3;-N: 2.8 mg/L (consent: 5 mg/L). BOD: 6 mg/L mean. No regulatory failures in 18 months post-commissioning.
Estimated saving versus new aeration tank construction:. MBBR intensification total cost:. Approved within AMP8 capital programme at 30% of the conventional upgrade option.
Fine bubble diffusers and DO cascade control reduced blower energy by 38%. Annual energy reduction: 420,000 kWh; equivalent CO&sub2; saving: 115 tonnes/year.
Modular MBBR design allows 50% future capacity expansion without new tanks, future-proofing the plant against population growth and tighter consent limits.
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