Les résultats des essais pilotes de technologies de traitement physico-chimique à haut débit (HRPCT) à New York (document en anglais)

PILOT TESTING OF HRPCT FOR CSO IN NEW YORK CITY

ABSTRACT

This paper presents the results of pilot testing of high rate physical chemical treatment (HRPCT) technologies in New York City at the 26th Ward wastewater pollution control plant (WPCP) during the summer of 1999. The US Filter/Microsep Ballasted Floc Reactor™ (BFR), the Actiflo™ from Krüger Inc. and the Densadeg 4D™ from Infilco Degremont Inc. (IDI) were evaluated by Hazen and Sawyer for the New York City Department of Environmental Protection (NYCDEP). The pilot units were treating primary effluent, which simulated combined sewer overflow (CSO) quality. Process performance was measured in terms of effluent concentration and percent removal for TSS and BOD5 as well as time to achieve full performance after start-up (start-up time).

Pilot testing was successful and showed that HRPCT is a compact technology that would be well suited for the treatment of CSO. All units performed well when operating correctly, removing between 69% and 84% of TSS on average. In addition, testing provided information on hydraulic loading, chemical dosage rates, recirculation rates, ballast dosage rate, operating ranges, as well as time to full performance after start-up and sludge quality. Some of the operational aspects should be further evaluated before proceeding to full-scale design.

KEY WORDS

HRPCT, CSO, wastewater, pilot testing, tube, plate, lamellar, high rate, settling, microsand, ballast, coagulation, flocculation, polymer, turbidimeter.

INTRODUCTION

Background

Typically, CSO control is achieved by constructing large underground concrete storage tanks. The stored CSO is pumped out of the tank to the local wastewater treatment plant after the rain subsides. Although CSO storage tanks have proven to be quite effective in reducing pollutant loading, their construction generally requires land acquisition and can be quite costly and disruptive to the surrounding community.

NYCDEP is considering using an innovative CSO treatment technology known as “high-rate physical-chemical treatment” or HRPCT. This technology has recently been implemented in Europe and was found to be highly effective in CSO pollutant removal there. Pilot testing of three leading HRPCT manufacturers was performed at the 26th Ward WPCP to obtain performance and operation data before considering a full-scale HRPCT facility in New York City. The three leading processes tested were the Ballasted Floc Reactor™ from Microsep/US Filter, the Actiflo™ from Krüger and the Densadeg 4D™ from Infilco Degremont.

Testing Setup

Each manufacturer’s equipment was tested individually for a one-month period for a total test period of three months. For each, mobilization and demobilization took about seven working days, such that total active testing was close to three weeks.

Several options were initially considered for bringing sewage to the pilot units. Ultimately, the least disruptive option for the main plant, while providing adequate influent simulating CSO, was chosen. Figure 1 shows the testing flowsheet.

Figure 1: HRPCT Testing Flowsheet

The Engineer provided an influent transfer tank where all three manufacturers installed their own pilot unit influent pump. The influent transfer tank also acted as an equalization tank because the location of both the discharge pipe and the influent submersible pump to the pilot unit induced excellent mixing and detention time in this tank was very short - about 2 minutes. Influent for testing was drawn using an electrical pump from the effluent end of primary tank no. 4. Therefore, the influent for the HRPCT pilot units was primary effluent that simulated TSS and BOD5 quality of New York City CSO adequately but contained very little floatable matter or grit. Influent solids to the pilot unit ranged from 20 to 150 mg/l. Additional solids were drawn during several runs from pass C of aeration tank no. 3 using a small sump pump, increasing influent solids up to 280 mg/l.

Process Characteristics

All units feature coagulant and polymer addition. In addition, HRPCT processes use “ballast”, which simultaneously helps floc formation and makes floc denser or heavier, thus allowing much higher settling rates than with traditional physico-chemical processes. Ballast recirculation and sludge wastage is between 5 and 13% of total flow. Process characteristics for all three units are summarized in Table 1.

Table 1: Process Characteristics of HRPCT Pilot Units

Process Characteristics Microsep Actiflo Densadeg

Ballast type Inert particle carrier (microsand) Microsand Own sludge

Ballast separation Hydrocyclone Hydrocyclone Gravity

Sludge extraction 2/3 of recirculation 2/3 of recirculation Not continuous

Mixing and maturation tanks 1 2 3

Settling tank Classic, no plates Lamellar plates Lamellar tubes

TESTING RESULTS

Overflow rates and detention times

Table 2 summarizes the range of overflow rates and flows tested during the study with the corresponding detention times.

Table 2: HRPCT Testing Summary

Unit no. of Runs

(start-up) Flow Rate (mgd) Hydraulic Detention Time (min) FeCl3

(mg/l) Polymer

(mg/l) Rise Rate

(gpm/ft2) Rise Rate

(gpd/ft2)

BFR 16 (6) 0.5-0.7 6-10 30-130 1.0-1.5 40-60 58,000-86,000

Actiflo 15 (8) 0.3-0.8 5-11 60-100 0.45-1.0 50-90 72,000–130,000

Densadeg 17 (5) 0.2-0.5 12-33 50-70 1.4-1.8 25-55 29,000-65,000

All HRPCT processes are extremely compact, with detention times between 5 and 33 minutes for all testing, as opposed to about two hours for traditional settling

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