INTRODUCTION

This process refers to the operation of activated sludge tanks in various configurations in a combination of

1) Suspended biomass,

MLSS (mixed liquid suspended solids) and
Attached biomass, which is attached to the media.

Such kind of operation is generally classified as Moving Bed Biofilm Reactor (MBBR).

Basic principle
Flock forming organism’s form clusters or attach to available surfaces. The media carrier material allows high biomass concentrations per cubic meter of material, which:

Increases the specific volumetric capacity of activated sludge tanks
Controls biomass activity
Reduces operating cost

The bar screen removes larger floating matter and suspended particles. Screened sewage flows into the STP tank, which contains the FBMR media. The FBMR media significantly increases the surface area for bacterial growth. Air is supplied through fine diffusers. Bacteria oxidize the organic matter present in the sewage. Oxidized sewage overflows in the lamella settler. Suspended particles in the treated wastewater settle, with a part of the settled sludge sliding back to the aeration tank. The clarifier Mechanism provides larger surface areas, thus reducing the settling tank size. Treated water overflows into a chlorine contact tank, wherein dosing hypochlorite solution through an electronic dosing system disinfects the treated water.

PROCESS DETAILS

Bar Screen:

Raw sewage from the source is usually received into the bar screen chamber by gravity. Screen provided will remove all floating and big size matter such as plastic bottles, polythene bags, glasses, stones, etc., which may otherwise choke the pipeline and pumps.

Oil and Grease Trap (Civil Construction)

If the sewage generated includes maximum quantity from kitchen and canteen, there is a possibility of higher concentrations of oil and grease in the raw sewage. It needs to be removed before biological treatment as it otherwise may cause problems for biological treatment. Usually, a small civil construction tank with a baffle wall and slotted oil pipe skimmer is provided. The oil and grease removed by gravity floats to the surface, which is removed by the oil skimmer (by client).

Equalization Tank (Civil Construction)

Usually, sewage generation is more during morning hours and evening hours. Visually no sewage is generated during night hours. Any biological system needs constant feed for bacteria to work efficiently. Hence, it is important to put an equalization tank to collect the excess flow during peak hours and feed sewage in lean hours. A typical equalization tank has a capacity of 8 – 12 hours of average flow rate. The tank is generally of civil construction by client. Provision of air grid is to be made for thoroughly mixing the sewage to make it of homogenous quality and to keep the suspended matter in suspension and to avoid septic conditions.

Transfer of Sewage

Our scope starts from transfer of sewage from Equalisation Tank to STP treatment tank. The distance of transfer should not exceed beyond 5 meter. The transfer pump can be either submersible or non-submersible type for this application. However we have considered centrifugal non-submersible type.

Fluid Media (FBMR)

Fluid Media (FBMR) as the name indicates consists of floating media of various shapes and sizes. The main objective of adding this media is to make available more surface area for bacteria to grow on, thereby maintaining and retaining maximum possible bacterial population in a limited volume. The FM media material allows biomass concentration of 20 – 40 Kgs/m3 material. Thus, FBMR consists of combination of biomass in attached as well as suspended form. High concentration of biomass enables reduction of aeration tank and in turn reduction in overall cost. Volume of the media shall vary from 6 to 25 % based on the concentration of organic matter.

Another main feature of the FBMR is its compactness. The FBMR consists of biological system for removal of organic matter (BOD, COD), lamella for clarification and chlorine contact tank for disinfection. As all units are placed inside a single tank, it saves space and also increases operational ease.

In FBMR, raw sewage enters at the top of the tank. Air is introduced at the bottom of the tank through fine bubble diffusers. Media will be in suspension because of the turbulence created by the air. The bacteria required for the oxidation of the organic matter is attached to the media and some part is suspended in the tank. After oxidation, the bacteria grow in number and need to be separated from the aeration tank liquor. The Clarifier section inside the FBMR helps in clarification and separation of the bacteria (sludge) and clear overflow flows into chlorine contact tank. Clarifier helps in increasing the settling area and removing the particles effectively in a smaller plan area. In chlorine contact tank, Sodium hypo Chlorite (NaOCl) is added for disinfecting the clarified sewage. Baffle are provided to make better contact. The chlorinated treated sewage then flows out of FBMR either for further treatment or for disposal.

Treated Water Collection Tank:

The treated water collection tank can be of civil construction by client in case required. The treated water can be collected either from the chlorination chamber in Scheme I or from Activated Carbon Filter in Scheme II.

Sludge:

The sludge from the Clarifier to be removed from the bottom of the Clarifier once in a day by client and transferred to sludge drying bed either by gravity or through pump depending on site condition. Before starting the Desludging, shut off the aeration for 30 minutes and allow the sludge to concentrate at the bottom.

After concentration, the bottom sludge valve can be open to drain the required quantity of sludge. An air scour pipe is provided to dislodge any media choking the drain outlet. Open the air scour valve for short time to remove the clog.