Liquid purification or separation – Processes – Treatment by living organism
Reexamination Certificate
2002-09-27
2004-11-30
Hopkins, Robert A. (Department: 1724)
Liquid purification or separation
Processes
Treatment by living organism
C210S660000
Reexamination Certificate
active
06824685
ABSTRACT:
TECHNICAL FIELD
The present invention relates to wastewater treatment, more specifically solids-liquid separation of activated sludge or concentration of excess sludge, particularly wastewater treatment processes and systems involving solids-liquid separation of activated sludge that can be used for the treatment of organic industrial and domestic wastewaters.
BACKGROUND ART
In the water treatment using activated sludge processes, activated sludge must be separated into solids and liquid in order to obtain treated water. A typical method for this purpose involved introducing activated sludge into a clarifier to settle the sludge by gravity and discharging the supernatant as treated water from the clarifier. However, this method required a clarifier having a settling area and a retention time enough to settle activated sludge, resulting in an increased size of the treatment system and a considerable space. When the settling properties of activated sludge were impaired by bulking or other causes, the sludge flowed out from the clarifier to invite deterioration of treated water.
In recent years, a method for separating activated sludge into solids and liquid by membrane separation in place of the use of a clarifier has also been used. In this case, a microfiltration membrane or ultrafiltration membrane is normally used as a solids-liquid separation membrane. However, this method essentially required suction or pressurization by pumping for filtration normally at a pressure of several tens to several hundreds of kPa, which led to a high pumping power and resulted in an increased running cost. Membrane separation had the advantage that a clear effluent free from SS (suspended solids) could be obtained, but also had the disadvantage that the permeation flux was low and that periodic chemical washing was needed to prevent the membrane from contamination.
More recently, an alternative method to the clarifier for solids-liquid separation of activated sludge was proposed, which comprises immersing a filter consisting of a gas-permeable sheet such as a nonwoven fabric in an aeration tank to give a filtrate at a low hydraulic head pressure. A general view of this method is shown in FIG.
1
. According to the proposed method, a diffuser tube for aeration
202
and a filter
204
are provided in a biological reaction tank
201
, and a diffuser tube
203
for air-washing the filter is placed below the filter. During biological reaction (filtration operation), raw water to be treated is supplied from a raw water feed pipe
205
into biological reaction tank
201
and air or the like are diffused from diffuser tube
202
to perform a biological treatment with activated sludge in the tank, and then the treated mixture of activated sludge and water is passed through filter
204
and the effluent (filtrate) is removed via discharge pipe
206
. During this, aeration by diffuser tube
202
generates a cross flow of activated sludge and water mixture (hereinafter also referred to as an activated sludge mixed liquor), including a downflow along the filter surface in biological reaction tank
201
(
FIG. 1
a
). This cross flow forms a dynamic filter layer of activated sludge on the surface of filter
204
, and the activated sludge mixed liquor is filtered by the resulting dynamic filter layer and removed via discharge pipe
206
. When the filter layer formed on the surface of filter
204
is consolidated to increase the resistance to filtration and decrease the filtrate flow rate, aeration from diffuser tube
202
is stopped and air is diffused from diffuser tube
203
to remove the filter layer on the surface of the filter by air-washing (
FIG. 1
b
). According to this method, a clear filtrate can be obtained by separation with a dynamic filter layer of sludge formed on the surface of the filter. The “dynamic filter layer of sludge” here refers to a deposit layer of activated sludge particles formed on the surface of the filter as filtration proceeds. The filter medium of the filter used in this method substantially has a pore size larger than that of activated sludge particles to allow the particles to pass, but a deposit layer of activated sludge particles (a dynamic filter layer of sludge) is formed on the surface of the filter medium under the condition of a low driving pressure for filtration so that this dynamic filter layer prevents activated sludge particles from passing through the medium. Filters commonly used in this method include nonwoven fabrics, woven fabrics, metal nets, etc. What is important in the method using the dynamic filter layer is to evenly and efficiently form a deposit layer of activated sludge particles as a filter layer on the surface of the filter medium with thickness, degree of compaction and other factors suitable for filtering activated sludge in order to reliably prevent the passage of activated sludge particles and to stably obtain treated water with good water quality. In the proposed method, it is defined that the dynamic filter layer is formed by controlling the flow rate of activated sludge flowing along the filter surface at an average of 0.05-0.4 m/s, preferably 0.15-0.25 m/s. In the proposed method, the filtration flux is about 2 m/d and the filtration duration is 2.5 h or more at a flow rate along the filter surface of 0.2 m/s, whereas the filtration flux is initially 4.1 m/d but rapidly deceases to 3.3 m/d after 45 min at a flow rate along the filter surface of 0.03 m/s.
A wastewater treatment system based on an activated sludge process was also proposed wherein a filter is immersed in at least one of a biological reaction tank and a final clarifier and the treated water is drawn from the filter via exits of the filter by the hydraulic head pressure difference from the succeeding tank.
However, these proposed methods had the following disadvantages. In the proposed methods, the flow of the sludge mixed liquor along the filter surface is formed by inducing a flow circulating in a tank by aeration. In these methods, however, the flow rate along the filter surface is not constant so that an even dynamic filter layer of sludge cannot be formed on the filter surface and the sludge readily deposits on the filter surface. Moreover, the water level in the biological reaction tank varies with the water inflow rate and the aeration air flow rate so that the hydraulic head pressure on the filter is not constant and the filtrate flow rate varies rather than remaining at a stable flow rate. If the hydraulic head pressure is unstable and extremely high, the water-permeability of the dynamic filter layer of sludge formed on the surface of the filter deteriorates, which may cause a sharp decline of the filtration flux. As a result, the washing frequency increases and the flux recovery rate after washing decreases. In addition, even minor residual organic contaminants such as BOD (biological oxygen demand) in the raw water entering into the biological reaction tank directly deposit on the filter and a biological slime grows on the surface of the filter to cause a remarkable decrease in filtrate flow rate.
When the filter is immersed in a final clarifier, the following problems occur. In the final clarifier using gravity settling of sludge, the sludge concentration in the clarifier is not even as shown by the fact that a thick sludge deposits at the bottom and the supernatant is collected from the top. Thus, the sludge concentration is uneven at the part where the filter is immersed, with the result that a good dynamic filter layer cannot be formed and a stable effluent cannot be obtained.
We further studied in detail the relation between the filtration flux of the filter and the surface flow rate along the filter in the filtration method using a dynamic filter layer of activated sludge mixed liquor, and found that, when the flow rate along the filter surface is 0.05-0.4 m/s, particularly in a preferred range of 0.15-0.25 m/s, the sludge on the filter surface vigorously flows to make it difficult to form an even dynamic filter layer of sludge with an effective filtration
Katsu Yosei
Tanaka Toshihiro
Ebara Corporation
Hopkins Robert A.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
LandOfFree
Method and apparatus for treating waste water does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for treating waste water, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for treating waste water will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3340419