ebsc

Posts about:

Biological Treatment

Biological Treatment, floatable solids, dition, aeration basin, biological waste treatment, bod, hydraulic overload, detention time

Primary Clarifier Operation

Primary clarification is the physical treatment process of removing solids before biological treatment. It is the most cost effective way to remove these solids after basic screening. Process water enters the clarifier tank and floatable solids (scum) are removed from the surface by skimmers while settleable solids (sludge) are collected on the bottom by a rake and removed via a sludge removal system. Effluent destined for biological treatment leaves the clarifier over a weir. The expected range for percent removal in a primary clarifier is 90%-95% settleable solids, 40%-60% suspended solids, and 25%-50% total BOD5.

Clarifier efficiency is based on hydraulic detention time, temperature of the water, the design of the tank and the condition of the equipment. Poor clarifier performance can be due to a variety of factors such as (1) hydraulic overload which decreases hydraulic detention time; (2) hydraulic under-load which doesn’t allow the equipment to work efficiently; (3) sludge buildup which causes decreased tank volume; and (4) highly concentrated waste streams. Bypassing a clarifier, which means routing plant effluent directly to secondary treatment in an aeration basin, should be done only in emergencies when clarifier equipment must be repaired or the sludge removal system is not able to process the sludge volume it receives. This scenario introduces high solids and elevated BOD levels into the biological treatment system and is not advised.

ebs
Read More
TSS | VSS, Activated Sludge Systems, biochemical oxygen demand, Biological Treatment

The importance of Primary Clarification: 

The Importance of Primary Clarification: A Key to System Efficiency

Well-functioning primary clarifiers are essential for wastewater treatment efficiency. However, they are often overlooked when diagnosing the causes of declining effluent quality. Their primary function is to remove insoluble BOD (such as fiber) and inorganic solids (like lime and ash), thereby reducing the load on the biological treatment stage. 

From a cost-saving perspective, primary clarifiers remove more BOD and TSS for less operational expense than any other treatment process, as rejected fiber from the process can contribute up to 0.4 pounds of BOD per pound of TSS. 

The diagram shows the difference between what enters the primary clarifier versus what leaves.

Green bars represent the TSS and BOD concentrations coming into the Unsettled Primary Clarifier Feed from the mill. If operators bypass the clarifier, this feed will go directly into the ASB.

Purple bars in the diagram represent the TSS and BOD concentrations in a typical Settled Primary Clarifier Effluent. These concentrations typically enter the ASB when the primary clarifier is functioning correctly.

Understanding Phantom BOD

Phantom BOD represents the insoluble portion of BOD that will not show up in the 5- day BOD test. This fraction will settle out somewhere in the ASB and break down to soluble BOD, creating additional oxygen demand later on. The primary clarifier significantly reduces the phantom BOD load to the ASB in this example. As seen in this diagram, a primary clarifier serves not only to reduce TSS but also a significant portion of BOD. Since more solids enter the ASB when bypassing the clarifier, bacteria will need time to acclimate to the higher BOD loading. There will likely be carryover of BOD and TSS into the effluent. Also, since more phantom BOD will be added to the ASB, the consequences of clarifier bypass may be seen weeks later in the treated effluent. 

Our experienced wastewater consultants help you closely monitor trends in primary clarifier performance, providing early detection of issues before they escalate. We also help ensure the biological portion of your treatment system remains resilient, even during periods when primary clarifier performance fluctuates!

Christina Dietzen
Read More
Activated Sludge Systems, continuous operation, lagoon system, Biological Treatment, industrial wastewater, Aerated Stabilization Basins, activated sludge system, cold weather, carbon source, aeration equipment, clari, inorganic matter

Pros and Cons of Aerated Stabilization Basins and Activated Sludge Systems

In North America, there are predominantly two systems used to biologically treat industrial wastewater: the aerated stabilization basin (ASB or aerated lagoon) and the activated sludge system. These two systems have far more similarities than differences. Both pro ­cesses are based on the utilization of dissolved oxygen by microorganisms in converting organic and inorganic matter into a settleable form. The following provides information on the pros and cons of each system.

Aerated Stabilization Basin – Figure 1

The ASB, also known as a lagoon system, is a large body of water used for biological treatment. The placement and continuous operation aeration equipment is both the most important part of the system and the most expensive. Untreated effluent comes into the primary clari­fier and then into the ASB. As the effluent passes through the system, the BOD or “food” present is used as energy and a carbon source for bacteria. Once the treated water has gone through the ASB it typically enters a polishing pond for tertiary treatment where the remaining TSS (total suspended solids) settles out before being discharged as the final effluent.  These systems are particularly widely used in the pulp and paper industry in the southeastern United States, where mills are usually remote and have access to large tracts of land.

ebs
Read More