Dated: this 27th day of March, 2020. , Description:FORM 2

THE PATENTS ACT, 1970
(39 of 1970)
COMPLETE SPECIFICATION (Section 10 and rule 13)

TITLE

“AN ABIOTIC PROCESS AND SYSTEM FOR PRE-TREATMENT OF WASTEWATER TYPE AS GENERATED FROM COKE MAKING IN STEEL PLANT”

APPLICANT

STEEL AUTHORITY OF INDIA LIMITED,
A GOVT. OF INDIA ENTERPRISE, RESEARCH & DEVELOPMENT CENTRE FOR IRON & STEEL, DORANDA, RANCHI - 834002, JHARKHAND, INDIA

FIELD OF INVENTION
The present invention relates to an abiotic process and system for pre-treatment of wastewater type as generated from Coke making in steel plant. More particularly, the present invention discloses abiotic treatment with a micro-bubble aeration system with micro bubble generation module (average bubble size of 50 µm) having a minimum flow rate of 10 litres/min and an aspirating aerator with Polypropylene rotor.

BACKGROUND ART
Chemical methods are very effective to prevent water pollution and remove specific pollutants. These methods employ chemicals added to the process fluid, usually solid or water based. These additives can control a large number of pollutants. However these are very expensive in the long run and additionally there are secondary effects (pollution as well as impact on carrying media) associated with these treatments. Besides, after preliminary reduction of pollutant concentration, efficacy of chemical treatment often gets reduced resulting in difficulty in achieving norms. With the high cost of chemical treatment and probably even higher cost of environmental chemical abatement, chemical use for water pollution control needs to be gradually reduced.

Biological treatment is the most cost effective as well as environmentally sustainable treatment process and is therefore widely adopted as the core treatment process in large number of industries including steel industry. However, compared to many other industries, effluent from steel industry, primarily coke oven effluent, is less biodegradable. This is because, presence of bio recalcitrant and/or toxic constituents often affect performance of biological treatment process/es. These bio refractory components are basically long chain organic compounds. Considering unsustainability of chemical processes, a coupled abiotic-biotic process is required which either increases effectiveness of biological treatment process or complement/supplement it as pre-treatment process.

SUMMARY OF THE INVENTION

AERATION- AN INTEGRAL COMPONENT OF BIO-TREATMENT

Aeration provides oxygen to bacteria for treating and stabilizing the wastewater. Oxygen is needed by aerobic bacteria to allow biodegradation to occur. The supplied oxygen is utilized by aerobic bacteria in the wastewater to break down the organic matter containing carbon to form carbon dioxide and water. Without presence of sufficient oxygen, bacteria are not able to biodegrade the incoming organic matter in a reasonable time.

OXYGEN TRANSFER-TRANSPORT MECHANISM

For aerobic biological processes, oxygen has to be transported from the gas phase to the microorganism in several steps (Fig 1). The largest resistance is the diffusion of oxygen through the liquid film around the bubble. This entails that oxygen is only slightly soluble in water. Oxygen transfer rate also depends on contact surface area and therefore coalescence and break-up, mass transfer coefficient, residence time, stability and concentration gradient

Therefore in order to maintain aerobic conditions, large quantities of oxygen must be provided. The primary function of the aeration system is to transfer oxygen to the liquid at such a rate that dissolved oxygen never becomes a limiting factor. Natural aeration cannot meet the demand of this high-rate unit process and therefore oxygen transfer must be engineered into the treatment unit in order to maintain a minimum residual of 1 mg of dissolved oxygen per liter of water.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Fig. 1 illustrates the steps of oxygen transfer and corresponding oxygen concentration in accordance with the present invention;

Fig. 2 illustrates the Dispersed Aeration for wastewater treatment in accordance with the present invention;
Fig. 3(a) illustrates the Mechanical Aeration- Turbine aeration system in accordance with the present invention;

Fig. 3(b) illustrates the Mechanical Aeration- Surface aeration system in accordance with the present invention;

Fig. 4 illustrates the Bubble rise velocity by size in accordance with the present invention

Fig. 5 illustrates the Macro and Micro bubbles in accordance with the present invention;

Fig. 6 illustrates the Bubble diameter vs rising time in accordance with the present invention;

Fig. 7 illustrates the Bubble diameter vs Shrinkage Time in accordance with the present invention;

Fig. 8 illustrates the Schematic Diagram of Laboratory Scale Air water interface contact area enhancement system through micro-bubble generation

Fig. 9 illustrates the Micro bubble aeration experimentation in accordance with the present invention;

Fig. 10 illustrates the Cyanide reduction during micro bubble aeration ( as pre-treatment ) in accordance with the present invention;

Fig. 11 illustrates the COD (Chemical Oxygen Demand) reduction during micro bubble aeration (as pre-treatment) in accordance with the present invention;

Fig. 12 illustrates the Cyanide reduction during micro bubble aeration (as post treatment) in accordance with the present invention.

DETAILED DESCRIPTION

Development of a technological approach combining abiotic and biological processes is envisaged for treatment of wastewater containing biorecalcitrant and/or toxic pollutants primarily generated in coke ovens of Integrated Steel Plants. Literature shows that even when a number of unit operations are used as pre-treatment, the key problem lies in soluble organic biorecalcitrant compounds which require application of chemical, biological and biochemical unit operations and processes, since single stage biological process/es are unable to achieve the targeted degradation efficiency.

It is postulated that as pre-treatment the abiotic treatment will reduce toxicity and produce biodegradable as well as lead to partial mineralization of pollutants. After literature survey, Microbubble aeration was identified as an abiotic process with potential since it is fully sustainable from the perspective that it does not add any chemical contaminant to the process fluid. Through experimental work it has been established that microbubble aeration can be coupled with biological wastewater treatment as pre-treatment for improved treatment and recyclability of organic effluent generated in coke oven a steel plant.

AERATORS

Aeration in wastewater treatment is mostly provided through subsurface (Fig. 2) and mechanical [Figs. 3(a) & 3(b)] systems. A subsurface system is where air is introduced into the wastewater by diffusers or devices submerged in the wastewater, usually at the bottom of the aeration tank. Air or pure oxygen bubbles are released at depth, producing a free, turbulent bubble-plume that rises to the water surface through buoyant force. As the bubble plume ascends, it entrains water, causing vertical circulation and lateral surface spreading. Oxygen is transferred to the water across the bubble interfaces as the bubbles rise from the diffuser to the top water surface. There are two main interfaces over which oxygen transfer occurs: across the bubble interfaces as the bubbles rise through the water column and across the water surface at the air-water interface. Mechanical systems, on the other hand, agitate the wastewater using mechanical devices, such as propellers and blades, to introduce air from the atmosphere.
OPPORTUNITIES FOR IMPROVEMENT IN EXISTING AERATION SYSTEM

This is now known that smaller the bubble, the greater the oxygen transfer rate. Additionally, bubbles formed deep within the chamber will have more hydrostatic pressure to drive the oxygen transfer and more time-of-contact with the air-water interface. However, in most biological wastewater treatment systems, aeration is provided using millimetre diffusers of various sizes and therefore limits the theoretical possibility of extending the effects of aeration. Micro-bubble aeration offers this possibility.

MICRO BUBBLE AERATION

Foremost significance of micro bubbles inside water compared to conventional aeration is their very low rising velocity. Microbubbles produced inside the surface of water present a different pattern of growth and collapse mechanism in comparison to macro-bubbles (range from millimeters to several centimeters). The increase in available surface area of the micro-bubbles for the same volume of a macro-bubble is significant as the bubbles decrease in size; an increased surface area is made available for the same volume. This therefore entails higher interfacial area. Besides, as bubble diameter decreases rising time increases thereby ensuring higher contact time.

OPPORTUNITIES FOR IMPROVEMENT IN EXISTING AERATION SYSTEM

The average size of the air bubbles is one of the important factors for minimizing the turbulence effect and decreasing movement speed (they take more time to reach water surface) and thereby improve effectiveness of aeration. In addition, lifting the suspended particles requires a laminar air bubble rise. This is now known that smaller the bubble, the greater the oxygen transfer rate. Additionally, bubbles formed deep within the chamber will have more hydrostatic pressure to drive the oxygen transfer and more time-of-contact with the air-water interface. One method of creating small bubbles is with porous ceramic diffusers. The small-interconnected passageways inside the ceramic matrix create a tremendous loss of air pressure and many points of outflow. This combination produces streams of small bubbles over the surface of the ceramic diffuser. However, in most biological wastewater treatment systems, aeration is provided using millimetre diffusers of various sizes and therefore limits the theoretical possibility of extending the effects of aeration. Micro-bubble aeration offers this possibility.

MICRO BUBBLE AERATION

Low rising velocity/Higher stability

Foremost significance of micro bubbles inside water compared to conventional aeration is their very low rising velocity. Microbubbles produced inside the surface of water present a different pattern of growth and collapse mechanism in comparison to macro-bubbles (range from millimeters to several centimeters). It has been reported that the bubble size and rise velocity share a direct proportionality which is ascertained by Stokes equation of rise velocity. Rising velocity, as represented by Stokes law is given by:

where ?l is the density of liquid, ?g is the density of the gas, g is the acceleration due to gravity, d is the equivalent bubble diameter and µ is the dynamic viscosity of the liquid. However the formulation is characterized by assumption which considers every bubble to be an isolated sphere devoid of collisions and coalescence during its ascension. However, even after taking this factor into consideration, conformable relationship between theoretical and experimental rise velocities has been reported in literature. A graphical comparison of rising velocity of different sizes of bubbles is as shown in Fig. 4.

Large interfacial area

The increase in available surface area of the micro-bubbles for the same volume of a macro-bubble is significant and this can be visualized by computing the ratio of surface area to volume of a perfect sphere as follows: (assumption that every bubble is a perfect sphere)

This relation lays emphasis on the fact that as the bubbles decrease in size, an increased surface area is made available for the same volume as represented in Figure 5. This therefore entails higher interfacial area. Besides, as bubble diameter decreases rising time increases (Fig 6) thereby ensuring higher contact time.

Bubble growth and shrinkage

Every bubble is characterized by a critical radius. As per simplified Young-Laplace equation, bubbles smaller than this radius tend to decrease in size and vice versa. (Fig 7) During coalescence when bubbles merge, the newly formed bubble radius becomes larger than the critical radius and inward diffusion of gas occurs as bubble begins to grow and becomes a macro bubble. Microbubbles have a very low rising velocity which increases the residence time inside the water or appropriate liquid.

ABIOTIC SYSTEM STUDIES

For abiotic experiments a microbubble aeration system was designed and installed at RDCIS as shown in Fig 8 which is self explanatory (timer based control)

In the system feed is introduced from an existing tank through a feed pump and valve to a mixing tank where micro bubble generation system is used for treatment of wastewater supplied. There was also a facility to treat the wastewater using an aspirating aerator mounted in an existing water tank. The key components of the system were Micro bubble generation module (average bubble size of 50 µm) having a minimum flow rate of 10 litres/min and an aspirating aerator with Polypropylene rotor. Fig 9 is the visual depiction of microbubble aeration during experimentation.

Experimental aspects

Conventional aeration can remove or oxidize a large number of chemicals, specially volatile organic chemicals. However, the process is extremely slow. It was postulated that Microbubble aeration offers the opportunity to accerate this oxidation process. Therefore at the core of this patent proposal lies the fact that whereas conventional aeration is primarily used as an aeration agent for oxygen supply, micro bubble aeration may be used as a pre-treatment agent. Laboratory scale experiments at RDCIS have shown that one no. of generator (less than 50 micron size bubble generated) is highly effective for pre-treatment of around 200 litres of wastewater for Coke Oven effluent. However, as control volume expands (i.e, distance from bubbler) effectiveness reduces.

Specific aspects:

Bubble size: 40-60 micron
Area coverage/ Control volume: 200 litres for each generator
Treatment time: 2.5 -3 hrs
Mode: batch recirculation
Bubbler location: Central to control volume
Conductivity: 300-500 µs
pH: 7.5-8.0

Figs 10, 11 & 12 shows the key results showing treatment effectiveness of

Comparison with conventional aeration

Micro-bubble aeration
Pre-treatment
Parameter % reduction Time Taken
COD More than 25% 2.5 hrs
Cyanide More than 50% 2.5 hrs
Post treatment
COD More than 20% 3 hrs
Cyanide More than 50% 3 hrs

Conventional aeration
Pre-treatment
Parameter % reduction Time Taken
COD More than 25% More than 72 hrs
Cyanide More than 30% More than 72 hrs
Post treatment
Negligible effect

• Micro-bubble aeration as abiotic process is significantly more effective than conventional aeration in reducing biorecalcitrance of coke oven effluent, especially in terms of reducing cyanide content.

• The effective degradation of cyanide could also be achieved through micro-bubble aeration in significantly less time than required for conventional biological treatment.

• Microbubble aeration as abiotic process cannot fully replace biological treatment since reduction of Ammoniacal-Nitrogen require application of biological treatment.

• Combined micro-bubble aeration with conventional biological treatment as pre-treatment/post-treatment can play the role of truly sustainable process for coke oven effluent treatment.

Salient features of innovation:
• A process for pre-treating coke oven effluent wherein driving force for treatment is microbubble aeration.

• A process where aeration through micro bubbling has been achieved as a treatment agent and not oxygen supply mechanism alone.

• A process where simultaneous higher level of aearation and chemical degradation can be achieved when combined with biological treatment

• A Coke oven effluent pre-treatment process where oxidation of target constituent is achieved without dosing of any conventional chemical or coagulant.

• A Coke oven effluent pre-treatment process where significant removal of normative constituents (e.g., say, cyanide) has been achieved without chemical or biological treatment

• An improved effluent pre-treatment process where improved oxygen supply is achieved compared to conventional aeration

• A system for effecting microbubble aeration for effluent pre-treatment

• A Coke oven effluent pre-treatment process where some process parameters for pre-treatment of effluent has been identified and established.

Although the foregoing description of the present invention has been shown and described with reference to particular embodiments and applications thereof, it has been presented for purposes of illustration by way of examples and description and is not intended to be exhaustive or to limit the invention to the particular embodiments and applications disclosed. The particular embodiments and applications were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such changes, modifications, variations, and alterations should therefore be seen as being within the scope of the present invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.

WE CLAIM:

1. An abiotic process for pre-treatment of wastewater as generated from Coke making in steel plant comprising the steps of :
introducing micro bubble aeration to the said wastewater from the bottom employing a Micro bubble generation module;
wherein the said aeration is done by passing air / oxygen gas with average bubble size of 50 µm and having a minimum flow rate of 10 litres/min and an aspirating aerator with Polypropylene rotor.
2. The abiotic process as claimed in claim 1, wherein the said air or pure oxygen bubbles are released at depth, producing a free, turbulent bubble-plume that rises to the water surface through buoyant force.
3. The abiotic process as claimed in claim 1, wherein as the bubble plume ascends, it entrains water, causing vertical circulation and lateral surface spreading.
4. The abiotic process as claimed in claim 1, wherein the said air / oxygen is transferred to the water across the bubble interfaces as the bubbles rise from the diffuser to the top water surface.
5. A system for pre-treatment of wastewater as generated from Coke making in steel plant comprising of:
a subsurface system where air is introduced into the wastewater by diffusers or devices submerged in the wastewater, usually at the bottom of the aeration tank wherein with average bubble size of 50 µm and having a minimum flow rate of 10 litres/min is released.
6- A system for pre-treatment of wastewater as claimed in claim 5, wherein the said air or pure oxygen bubbles are released at depth, producing a free, turbulent bubble-plume that rises to the water surface through buoyant force and as the bubble plume 13
ascends, it entrains water, causing vertical circulation and lateral surface spreading and
wherein the oxygen is transferred to the water across the bubble interfaces as the
bubbles rise from the diffuser to the top water surface.