Ⅱ Sewage treatment method (for ST/RT type development)

The history of our municipal sewage treatment plants began more than 200 years ago in England, and the technology was transferred to Japan. Initially, sewage entered the treatment plant through sewer pipes in England and was treated with a process called activated sludge, which is a process in which oxygen is added to the aeration tank or reaction tank to break down organic matter through microorganisms under aerobic conditions.

The sludge treated by this process was relatively well settled, with concentrations of over 10,000 mg/L and 11 TP3T. These sludges were sufficiently dewatered by a system that adds polymer flocculants to the sludge and dewaters it. ARK is applying RT type for such sludge with good settling, and below [Figure] is a system of flocculation in flocculation-mixing tank and dehydration in dehydrator with ARK RT type.

[ Figure 2 ] ST-Type

In Europe, septic systems that separate sewage are not common due to low rainfall and sparse housing. However, in Japan and South Korea, the sewage collection system only collects sewage, resulting in high concentrations of sludge. Due to this high concentration, nitrification of sludge naturally occurs in the summer when the temperature is high, and a process to treat T-N through natural denitrification was introduced.

Since such treatment methods utilize light nitrate-producing microorganisms, the concentration of the sludge is reduced and lightened due to poor sedimentation, which makes it difficult to directly dewater the sludge, which is about 7,000 mg/L, 0.71 TP3T or less, and even 4,000 mg/L or less, making it difficult to directly dewater with conventional belt dewatering machines and centrifugal dewatering machines. To solve this problem, the method of placing the concentration facility in the front and dewatering after concentration has become a common phenomenon in Japanese and Korean sewage treatment plants.

ARK has developed a dehydrator that can concentrate and dehydrate such low-concentration sludge in a batch dehydration method without installing a separate concentration facility, and it is the ST system.

[ Figure ST Type ]

In addition, this ST-Type automates the dewatering system by dewatering surplus sludge at a low concentration of about 2,000 mg/L directly from the wastewater tank, and reduces installation costs by eliminating thickening tanks, retention ponds, and odor facilities.

[ Figure Conventional dehydration method ].

[Figure ST-Type Treatment Plant Application Example].

In conclusion, the ARK-RT system is recommended for sludge concentrations above 1.01 TP3T and the ARK-ST system for sludge concentrations below 11 TP3T.

1) RT-TYPE

The structure and principle of the RT-Type, which is suitable for relatively high concentration sludge, consists of a flocculation mixing tank and a dewatering system. Sludge enters the flocculation tank inlet through a pipe ([Figure 1]-①) from the sludge reservoir to the sludge transfer pump as shown in Figure 1.

Only a certain amount of the sludge transported to the flocculation tank inlet is sent to Part 2, and the remaining sludge is U-turned to the treatment plant through the piping ([Figure 1]-②).

In Part 2, the constant polymer transported through the piping ([Figure]-③) using the polymer pump as shown in [Figure 1] and the sludge introduced into the lower part of the flocculation tank ([Figure 1]-④) through Part 1 are stirred by the agitator installed in the upper part of the flocculation tank.

The sludge entering through the lower part of the flocculation mixing tank is then separated into flux and filtrate by the action of the polymer. After being separated into flux and filtrate, it enters the dehydration facility, i.e., the dehydrator, through the input pipe ([Figure 1]-⑤).

[ Figure 1 ] RT-Type

2) ST-TYPE

The ST-Type, which is suitable for low concentration sludge, consists of a thickening unit and a dewatering unit. The sludge is concentrated in the thickener and then sent to the dewatering unit.

As shown in the figure below, the sludge is piped from the sludge storage tank to the sludge transfer pump ([Figure 2]-①) to the bottom of the thickening plant.

The sludge entering the lower part of the thickening facility is moved by the lower impeller installed at the bottom, where the sludge and chemicals are mixed and gradually moved to the upper part, separated into flux and filtrate, and moved in a solid and large flux ([Figure 2]-②).

At the top of the concentrator, the upper impeller generates a pre-rotating current to grow a larger flux ([Figure 2]-[3]).

The filtrate outlet installed at the top of the center of the thickening plant excludes the filtrate separated into filtrate and flux through the filtrate outlet and enters the treatment plant ([Figure 2]-[4]), and the concentrated sludge is fed to the dewatering plant ([Figure 2]-[5]).

[ Figure 1 ] ST-Type

1) Select TYPE

• Select RT-TYPE for sludge concentrations above 10,000 mg/L (1%)
• Select ST-TYPE when sludge concentration is 10,000 mg/L (1%) or less

2) Set up zones based on sludge type

Since the dewatering capacity varies depending on the sludge characteristics, the sludge area corresponding to each treatment plant is selected according to the graph and description below.

[ Change in water content with VS/TS ].

Ⅵ Calculate solids volume

1) Calculate solids volume

The most important factor in model selection is solids volume. Solids volume is calculated as the product of flow rate and concentration, and is a function of the solids
The amount is used to select the appropriate model. The solids selection method is as follows.

Solids Volume (kg-DS/hr) = Flow Rate (m3/hr) × Concentration (mg/L)

-Solids selection example

[ when flow rate, concentration is known ]

[ cake yield, water content ].

Ⅴ Throughput and model selection based on VS/TS and utilization

The processing capacity of the dehydrator varies depending on VS/TS and moisture content. Therefore, the solids volume obtained in 3-2-3 is applied to the organic content (VS/TS) by sludge area (A, B, C, D) and the processing capacity of each model according to the target moisture content to select a model.
(If it doesn't match the solids volume listed in the throughput by target moisture content, you can select a TYPE with a higher throughput than the solids volume.)

Ⅵ Device selection examples

Example of model selection when sludge inflow concentration is 2.51 TP3T or less

1) RT-Type

Design conditions

• Influent sludge concentration: 15,000 mg/L (1.51 TP3T)
• Inflow flow rate : 30㎥/day
• Dehydrator Uptime: 8hrs/day
• Solids : 30(㎥/day) ÷ 8(hr/day) × 15,000mg/L = 56.25kgDS/hr

Select a device by area

⊙ A-zone (VS/TS 85% or higher, when target function rate is 80%)
[Refer to Table A-1 and select the type that corresponds to a solids volume of 56.25 kgDS/hr.