COMBI-TREAT IMPROVED SBR TECHNOLOGY

Conventional SBR is improved by introducing Combi-Treat Unit ahead of CLSBR (Constant

Level SBR) Unit to generate power from Bio-gas and to reduce organic load in CLSBR basins. I-

SBR technology has potential to generate power from two sources:

• Power from Biogas through Combi-Treat unit by providing Bio-Gas engines, and

• Power from Hydraulic head available for treated sewage while flowing to discharge.

It has been established that I-SBR Technology saves 70% power in comparison with

conventional SBR technology considering both power generation and saving in power due to

reduced size of blowers..

Detail analysis about saving in power is shown in Scenario – 1 and 2 separately under Chapter

IV & V.

2.1Process Diagram:

2.2 Process Variation

The Improved SBR Technology is a result of one of the fundamental principles of HNBEPL,

“Continuous Improvement”. After constructing the first STP on Conventional SBR technology

first of its kind in India at Goa of 12.5 MLD capacity, HNBEPL has designed, engineered &

constructed many other plants on Conventional SBR technology and few more are under

execution.

In the Improved SBR technology, conventional SBR technology has been modified to enhance

its performance in terms of Power Savings. Certain additions (for which patents are pending)

have been accommodated in the conventional technology to generate power in generous

proportions. Improved SBR technology is one of a kind technology that provides tangible

benefits throughout considerable life of the STP but generation of power by two means:

• Power Generation from Bio Gas

• Power Generation from Hydrostatic Head

COMBI TREAT UNIT (PATENT PENDING):

Brief Function: This revolutionary unit is an improvement made to the Conventional SBR

technology to convert into power saving and power generation technology, which modifies the

process to Improved SBR technology. This innovation has incorporated the advantages of both,

aerobic and anaerobic sewage treatment methodologies. Sewage is treated in this Combi-treat

unit before it enters the I-SBR basin. Combi-treat unit consists of a large tank, preferably

cylindrical in shape. Upper portion of Combi-Treat functions as Primary Clarifier and bottom act

as Anaerobic Sludge Digester. Anaerobic digestion is the biological degradation of organic

matter in the absence of free oxygen. During this process, much of the organic matter is

converted into methane, carbon-di-oxide and water and therefore the anaerobic digestion is a

net energy producer. There is a dome at the top of the tank to store gas produced from Digester.

The collected gas is then scrubbed to remove impurities and moisture. Further a gas engine

facilitates power generation from Bio Gas.

Process Description: After screening and de-gritting unit, raw sewage enters to Combi -treat Unit

where much of the organic matter settles, like it does in a primary clarifier. Proper feeding

arrangement of raw sewage into Combi-Treat Unit enables to settle the sludge. Sludge is

allowed to settle at bottom of Combi-Treat Unit, which act as Sludge Digester. Draft tube type

mixers are provided to mix entire sludge settled at bottom in such a way that settled raw sewage

on upper portion of Combi-Treat Unit does not affect. Thorough mixing helps in stable

performance of the digestion process and creates a homogeneous environment throughout the

digester. It also quickly brings the raw sludge into contact with microorganisms. Furthermore,

when stratification is prevented because of mixing, the entire digester is available for active

decomposition, thereby, increasing the effective solids retention time (SRT).

This Combi-Treat Unit ensures BOD removal efficiency around 35% to 45% and SS removal

efficiency around 45% to 60%. Removal of these parameters in Comi-Treat Unit results in less

power requirement for balance organic load in the I-SBR Basins.

The supernatant from upper portion of Comi-Treat Unit is allow to flow over a weir and flows

radially outwards into CLSBR Basins.

CLSBR (Constant Level SBR)

The supernatant from Combi-treat Unit flows into external basins that work on the principle of

Constant Level SBR. The CLSBR basins act as aeration tanks as well as secondary clarifiers.

By applying air through diffusers into the CLSBR tanks, sewage is treated aerobically. Sludge is

settled and removed using pumps.

System is operated in a batch mode in sequence which eliminates all the inefficiencies of the

continuous processes. A batch reactor is a perfect reactor, which ensures 100% treatment.

Three modules are provided to ensure continuous treatment and the flow is distributed by using

motorized gates at inlet chamber of basin operated on PLC. The complete process takes place

in a single reactor, within which all biological treatment steps take place sequentially. The

CLSBR basins are equipped with air blowers, diffusers, Waste Activated Sludge (WAS) pumps,

Decanters Gates, Auto valves, PLC etc. All cycles will be automatically controlled using PLC.

Excess sludge at a consistency level of approx 0.8% - 1.0% will be pumped intermittently from

WAS pump to the sludge thickener.

The sludge from sludge thickener is taken for dewatering to centrifuge units and finally for its

ultimate disposal. The treated effluent from the CLSBR Basins will be conveyed to outfall near

drain for ultimate discharge. No additional settling unit / secondary clarifier are required. The

complete biological operation is divided into cycles. Each cycle is of 3 hrs duration, during which

all treatment steps take place.

Cyclic operation in CLSBR:

A basic cycle comprises:

• Decant-Fill (D/F)

• Aerate (A)

• Settlement (S)

A Typical Cycle

Decant-Fill Cycle: The supernatant after settling is disposed-off to further unit from the basin by

substituting equivalent quantity of raw sewage as Fill in the I-SBR Basin. Thus ‘x’ m3 of raw

sewage replaces to the ‘x’ m3 of treated sewage. In this way both Decant-Fill Cycle works.

During decanting there is inflow to the basin. Decanter gates opened after receiving signals from

PLC on start of filling raw sewage into basins. After the required volume of supernatant/treated

sewage is removed, the Decanter gate receives the signal from PLC to close down the gates.

The basin is now ready for the next cycle of Aeration to begin. Stainless steel fabricated

Decanter Gates ensures resistant to corrosion, long equipment life without any/no maintenance

Aeration Cycle: - The aeration system is equipped with high quality fine bubble membrane

diffusers. This system is of most important core part playing major role of saving power in this

technology. Online DO Meter controls DO in water and gives signals to PLC to on/off Air

Blowers to maintain predefined DO level in the water. Size of Air Blowers reduces drastically as

35% to 40% organic load is removed in Combi-Treat Unit and as such air requirement is only for

balance organic load.

Settlement Cycle: - After the aeration cycle, the biomass allow to settles under perfect settling

conditions. Surface area of the basin is selected in such a way that outlet parameter for SS

remains always ≤ 10 mg/lit. Once Settled the supernatant is replaced by fresh raw sewage,

which start Fill cycle. Solids are wasted from the tanks during the decanting phase.

The SBR Technology is configuration of activated sludge process which operates on extended

aeration of activated sludge. This works on the principle for BOD reduction, Nitrification,

Denitrification as well as biological phosphorous removal. This is equipped with energy efficient

fine bubble membrane diffused aeration system.

Fully PLC Based Control for Operation: - The complete operation of the CLSBR basin/reactor

is controlled automatically through PLC system, which is a major factor in reducing operating

costs. All key functions like, sludge wasting, aeration control, batch cycle time control, etc. are

automatically controlled along with data logging.

2.3 Process Chemistry

• Due to additional aeration for nitrification, oxidation of organic matter in waste water,

nitrification is enhanced.

• In anoxic zone de-nitrification of nitrates is achieved by recalculating activated sludge from

aeration zone. The retention period and recycle ratio is designed as per Metcalf & Eddy.

BOD removal and Nitrification in Aeration zone:

BOD removal:

The aeration zone of I-SBR/CLSBR basin is provided with diffused aeration system to oxidize

the organic matter by activated sludge present in the basin. The activated sludge in aeration

zone is capable of converting most organic wastes to stable inorganic forms or to cellular mass.

In this process, the soluble and colloidal organic material is metabolized by a diverse group of

microorganisms to carbon dioxide and water. At the same time, a sizeable fraction of incoming

organic matter is converted to cellular mass that can be separated from the effluent by settling.

Activated sludge comprises a mixed microbial culture wherein the bacteria are responsible for

oxidizing the organic matter, while protozoa consume the dispersed un-flocculated bacteria and

rotifers consume the unsettled small bio-flocs in the treated wastewater, performing the role of

effluent polishers.

The utilization of substrate by a bacterial cell can be described as a three-step process:

a. The substrate molecule contacts with the cell wall.

b. The substrate molecule is transported into the cell.

c. Metabolism of the substrate molecule within the cell

However, as the bacteria require the molecule in the soluble form, colloidal, spherically

incompatible molecules, which cannot be readily biodegradable, have to be first adsorbed to the

cell surface and hydrolyzed or transformed externally to transportable fractions by exo-enzymes

or wall-bounded enzymes. The organic matter will be utilized by the bacteria resulting in cell

synthesis and energy for maintenance. Nutrients available in the wastewater cater to the nutrient

requirements of the aerobic microorganisms and to enhance the activity of the aerobic microbes.

In addition to the nutrient requirements, the aerobic microbes require oxygen to sustain their

microbial activity. Oxygen functions as a terminal electron acceptor in the energy metabolism of

the aerobic heterotrophic organisms indigenous to the activated sludge process. In other words

a portion of the organic material removed is oxidized to provide energy for the maintenance

function and the synthesis function.

The following reactions best describe the organic substrate utilization by the aerobic bacteria:

Oxidation

COHNS + O2 -------------� CO2 + NH3 +C5H7NO2 + Other end products

Organic matter Bacteria New cells

Endogenous respiration

C5H7NO2 + 5O2 ----------� 5CO2 + 2H2O + NH3 + energy

New cells Bacteria

Synthesis

COHNS + O2 + Bacteria � C5H7O2N (New Bacterial Cell)

It is to be noted that the activated sludge in I-SBR/CLSBR Basin operates in extended aeration

mode. An extended aeration activated sludge process operates in the endogenous respiration

phase of the growth curve where the microorganisms are forced to metabolize their own

protoplasm due to the limited availability of food or substrate. During this phase, the nutrients

remaining in the dead cells diffuse out to furnish the remaining cells with food. This system has

been developed for application where minimum bio-solids production is desirable. Less solids

production is achieved by using a larger fraction of the entering organic material for energy

rather than for synthesis. This means that more oxygen will be consumed per unit mass of

organic material removal.

Nitrification

I-SBR/CLSBR system, with high sludge retention time (θc) and DO > 2.0 mg/L ensures uniform

nitrification. Nitrification results from the oxidation of ammonia present in the sewage by

Nitrosomanas to nitrite and the subsequent oxidation of the nitrite to nitrate by Nitrobacter. The

nitrifying organisms are strict aerobes and require more than 2 mg/L DO in the I-SBR/CLSBR

basin to avoid oxygen limitation. The nitrification of ammonia can be represented as given

below:

2 NH4+ + 3O2 2NO2

- + 2 H2O + 4 H+ + New Cells

2 NO2

- + O2 2NO3- + New Cells

The diffused aeration system is sized in such a way that sufficient oxygen is provided for

carbonaceous oxidation, sludge stabilization, nitrification by maintaining the DO at the specified

level of 2 mg/L. The capacity of diffused aeration in each basin will be sufficient to ensure good

and uniform mixing conditions during Aerate phase of the cycle of operation.

De-nitrification, in Anoxic Tank

The wastewater enters into the anoxic Tank from the front end of the basin / reactor, where

anoxic conditions are maintained. In addition, return sludge from the aeration zone is also

recycled by using recirculation pumps. As microorganisms meet high BOD, low DO conditions in

the anoxic zone, natural selection of Phosphate Accumulating Organisms (PAOs) and floc-

forming microorganisms takes place. This is very effective in containing all of the known low F/M

bulking microorganisms and eliminates the problems of bulking and surface foaming. Also, due

to the anoxic/anaerobic conditions in the anoxic tank, de-nitrification and phosphorous release

occurs.

The process of de-nitrification of nitrates is represented as:

NO3- + BOD � N2 + CO2 + H2O + OH- + cells.

De-nitrification releases nitrogen which escapes off as an inert gas to the atmosphere.

NITROSOMONAS

NITROBACTOR

2.4 Advantages of Improved SBR technology

• Combi-treat unit replaces both primary clarifier and anaerobic digester

• Methane is generated from Combi-treat unit which can be collected using floating or flexible

dome. The gas may be used to operate gas engine and thereby produce electricity

• As substantial organic matter is ‘settled’ in Combi-treat unit, lesser organic loading is to be

handled in the outer CLSBR basins. Power Saving occurs on the account of smaller Blowers

required to handle these lesser organic loads

• CLSBR basins replace aeration tanks and secondary clarifiers

• Not only is there a saving in power on account of smaller blower requirement but power is

also generated by gas engine and hydropower turbines Entire Plant is PLC operated.

• Plant can be completely controlled from a remote location using internet. Easy operation &

maintenance.

• No foaming problems

• Excellent outlet parameters. Guaranteed performance: BOD <10 mg/l and TSS<10 mg/l.

[Actual Performance BOD < 5 mg/l &TSS < 5mg/l]

• It saves 60% power required in comparison with conventional SBR technology.

• Energy can be produced from both sources viz. Biogas and hydropower

2.5 Improved SBR Technology - 30 MLD STP at Akurdi

We have already constructed a 30 MLD capacity Sewage Treatment Plant at Akurdi, Pune on

Improved SBR technology which is recently commissioned. Plant is continuously delivering

outlet parameters for BOD5 @ 200C and SS as ≤ 10 mg/lit. Preliminary results show remarkable

power savings and good quality bio gas is produced, which can be used for power generation.

Moreover hydro-turbines are being installed to generate hydropower while flowing treated

sewage from I-SBR/CLSBR to CCT. All this is achieved without hampering the outlet water

quality parameters. Following are the various photographs of 30 MLD STP on I-SBR technology

at Akurdi.

PHOTO: AERIAL VIEW OF 30 MLD STP AT AKURDI ON I-SBR TECHNOLOGY

PHOTO: VIEW OF MEMBRANE GAS DOME OF 30 MLD STP AT AKURDI ON I-SBR TECHNOLOGY

PHOTO: VIEW OF SLUDGE HANDLING UNIT OF 30 MLD STP AT AKURDI ON I-SBR TECHNOLOGY