Flocculation-Flotation vs. MBR for High MLSS Secondary Clarification
Abstract
Rendering plants produce high strength wastewater with high TSS, FOG, BOD and TKN. At one such plant it was decided to install a primary DAF followed by an anaerobicanoxic-aerobic treatment system for TSS, FOG, TKN and BOD removal. Since wastewater is a high strength (TSS up to 10,000, FOG up to 3,000, TKN up to 350 and BOD up to 6,000 mg/l, respectively, bioreactors will produce high MLSS up to 11,000 mg/l. Pilot scale studies tested the feasibility of using either flocculation-flotation or membranes for MLSS removal. Only a small space was available inside the building, making it necessary to operate solid/liquid separation at an HRT of 5 GPM/ft2 , and a solids loading of up to 15 lb/ft2 /hr. Currently engineers consider that it is impossible to clarify aerobic MLSS at loading over 10 lb/ft2 /hr unless membrane separations are used. After the pilot studies where solid/liquid separations were achieved with the GEM flotation System, full scale System was installed in 2012 and is operating for the last three years. This manuscript describes the pilot study and full scale installation of the GEM flotation System at this plant.
Introduction
Goals and Objectives
The primary goal of this study was to compare the performance of membranes and a small footprint advanced flotation system for high MLSS clarification. After the pilot study, a full scale wastewater treatment system was to be built at a California rendering facility.
The Pilot Study
Membranes and an advanced small footprint GEM flotation System were tested at a Wahoo, Nebraska rendering facility. Return activated sludge, RAS (5%), was diluted to different concentrations of MLSS to use as a model system for high MLSS between 3,000 and 17,000 mg/l.
Membranes (flat sheet microfiltration from Kubota) were able to remove up to 17,000 mg/l of MLSS. However, high amounts of calcium (350 mg/l) and alkalinity in the water resulted in very fast calcium carbonate scaling, and an unacceptably frequent need for membrane acid cleaning (once a week). This would result in the replacement of membranes of at least once a year.
The GEM System (advanced in situ flocculation - flotation described later in the text) was able to remove between 3,000 and 11,000 mg/l of MLSS at HRT of 5 GPM/ft2 . Cationic high molecular weight high charge (55%) granular polyacrylamide flocculant C- 498 HMW from KEMIRA (available in GRAS form) had to be used for the best clarification and sludge thickness (4.5-6.5 % of dry solids). After the GEM System, effluent TSS was on average 20 mg/l, FOG 0 mg/l and BOD 22 mg/l - with TKN of 12 mg/l. Sludge depth inside the tank varied between 1 inch and 3 feet. At MLSS above 12,000 mg/l and HRT of 5 GPM/ft2 [loads above 16 lb/ft2 /hour] sludge depth was 6 feet, which resulted in a large amount of carryover inside the effluent. Including safety factors, the GEM System can be used at an HRT of 5 GPM/ft2 to remove up to 11,000 mg/l of MLSS from an aerobic reactor. The flocculant dosage at 11,000 mg/l was 90 mg/l of C-498 HMW.
The polymer dosage did not interfere with the performance of the RAS. RAS was mixed with the incoming wastewater with the maceration pumps to break down strong flocs produced with the cationic polyacrylamide flocculant. High calcium concentrations did not interfere with the flocculation - flotation process. No scaling occurred at the tank surfaces due to high enough flow rates. Since the sludge’s dry weight was between 4.5 and 6.5%, no additional thickening was necessary (see Figure 1 for a picture of sludge skimmed from the GEM System installation). Dewatering in decanter centrifuges was easily achieved without the addition of further flocculants. Produced dry sludge could be used for composting or landfill disposal. Jar tests have to be performed and chemistry dosages adjusted to achieve the best separation efficiencies. The pH of the influent was around 7.4 and did not have to be adjusted. TDS of the influent was around 1,200 mg/l. As it is described below, the GEM System is a hybrid centrifugal - dissolved air flotation system where the mixing energy of flocculants and particles can be adjusted. Moderate mixing energies inside the liquid hydrocyclones (LCPP’s - liquid cyclone particle positioner) were used (see below for description of the GEM System). Flocculants were added after the aeration-cavitation plate to avoid floc breakage and polymer chain breakage.
Various polymeric flocculants were tested. The higher the molecular weight, the better stronger flocs were produced. Flocculants with more charge also produced better sludge at lower dosages. Granular flocculants always outperformed emulsion flocculants. When high dosages of cationic flocculant had to be used (60-100 mg/l), better results were achieved if flocculants were added in two separate LCPP`s. Better results were achieved when flocculants were prepared at 0.25 % than at 0.5% (due to viscosity and mixing issues).
Figure 1 – Sludge Skimmed from GEM System at Rendering Facility