Yorkshire Water Services


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Introduction

This project with a value of £2.5 million was a contract to design, build and commission four effluent treatment plants treating the liquors from gas cleaning systems at Yorkshire Water Services' sewage sludge incinerator sites at Knostrop (Leeds), Esholt (Bradford), Blackburn Meadows (Sheffield) and Calder Valley (Huddersfield). The order placed by Earth Tech Morrison included the supply of all items of automated equipment, mechanical and electrical services, instrumentation and control.

These plants were designed to treat contaminated scrubbing liquors from four upgraded gas cleaning systems, the new effluent treatment plants were part of a Yorkshire Water initiative to ensure full compliance with latest European Waste Incineration Directives. The effluent treatment plants satisfy European legislation, EC 2000/76 limits, with particular emphasis on the removal of suspended solids, mercury and other heavy metals.

Design Information

The following table indicates the Acid / Alkali Influent quality and the final effluent quality achieved from these SET Plants.

Description Acid Influent
Characteristics
Average
Alkali Influent
Characteristics
Average
Average Effluent
Quality Achieved
Flow m3/h 4 4 8
pH 0.5 - 3.0 4.0 - 10.0 6 - 9
Temperature °C 60 - 80 50 - 70 45
Suspended Solids (mg/l) 10,000 30
Mercury (mg/l) 4.0 0.03
Cadium (mg/l) 0.8 0.05
Lead (mg/l) 0.6 0.2
Copper (mg/l) 3.0 0.5
Nickel (mg/l) 2.0 0.5
Zinc (mg/l) 5.0 1.5

Plant Description

At each of the four sites, ash particles are removed from the fluidised-bed incinerator flue gasses by an electrostatic precipitator installed before the wet scrubbing system. Re-circulated liquor in the first stage acid quench scrubber will remove dust, HCI and HF. Sulphur dioxide (SO2) and the bulk of the remaining dust and hydro halogens will be removed in the second stage packed column alkali scrubber.

To prevent any of the constituents absorbed by the washwater from increasing concentration to a level where it would interfere with the efficient operation of the scrubber, blow-down from each scrubber effluent treatment plant is transferred at a continuous rate of 4m?/hour.

The SET plants are be capable of operating round the clock, 365 days a year, so as not to inhibit the continuous operation of the incinerators. Each plant accepts blow-down liquor from the scrubbing system, removes contaminants and provide secure discharge to the sewage works drainage system.

The scrubber blow-down liquors, acid and alkali, are transferred from the scrubbers to the acid and alkali balance tanks at a pre-set flow rate of 4m3/hour. The two balance tanks are equipped with mixers, to prevent settlement of solids, and pH/temperature monitors. Both tanks provide 4 hours storage under normal operating conditions. The effluent from each tank is pumped via a common line to the neutralisation tank, and then flows by gravity to the precipitation tank. Both tanks are equipped with mixers. Within the neutralisation tank sodium hydroxide (caustic soda) is added to raise the pH to 7.5. In the precipitation tank, the pH is further adjusted to pH 8.5 by the addition of sodium hydroxide. This is the optimum pH for the removal of mercury and other heavy metals using TMT15.

TMT15 is a proprietary additive, specifically used for the removal of mercury and heavy metals, for example, nickel, lead and copper. TMT15 is dosed, at a fixed rate, to the precipitation tank, where it precipitates out the mercury and metals in solution. Ferric chloride and a polyelectrolyte are also added to the precipitation tank at fixed dose rates. The ferric chloride flocculates the TMT/heavy metal precipitate and polyelectrolyte is used to aid settlement.

For operational flexibility dosing lines for TMT15 and ferric chloride into the neutralisation tank were also installed. The effluent from the precipitation tank flows by gravity to a settlement tank. The flocculated solids and metals settle to the base of the settlement tank, from where they are removed to a sludge storage tank. The clarified effluent overflows to a collection tank, and transferred to a sand filter for further treatment.

A continuously operating upward flow sand filter is used to remove the remaining traces of fine solids and metal precipitate to ensure the discharge consent is met. The filtered effluent gravitates to the discharge point, via a monitoring station for pH, temperature and flow. An autosampler installed in the discharge line takes a representative sample for laboratory analysis.

The backwash water containing the removed solids particles is returned to the settlement tank.

Settled solids from the settlement tank are pumped to a sludge storage tank. The storage tank is equipped with a picket fence thickener and decant tree to promote consolidation before thickening.

Sludge from the storage tank is pumped to a centrifuge, with polyelectrolyte added to the centrifuge feed to assist the dewatering process. Sludge cake, with dry solids content in excess of 30% is produced, and disposed of off-site. The centrate liquors are returned to the alkali balance tank for reprocessing through the system.

The sodium hydroxide required for pH correction is taken from a ring main, which also feeds the sludge incinerator plant. Addition of the sodium hydroxide is via a motorised valve, which is controlled on a pH set-point. A pH meter is installed in both the neutralisation and precipitation tanks.

TMT15 and ferric chloride are stored in bulk tanks. Duty/standby dosing pumps deliver the chemical at a fixed rate to the precipitation tank.

A liquid polymer preparation system provides diluted polyelectrolyte, at the required concentration, to the precipitation tank for settlement purposes, and to the centrifuge to aid dewatering. Polyelectrolyte is added, at a fixed rate, using a dedicated duty/standby dosing set for each process.

Project Details

Client: Yorkshire Water Services

Industry: Sewage Sludge Incinerator Liquors

Application: Scrubber Effluent Treatment (SET) Plants

Technologies

Wastewater Treatment
Scrubber Effluent Treatment Plant