The activated sludge process

1. Introduction.

Within the wastewater treatment options, one of the most commonly used ones is the activated sludge process. In the previous article, a comment about this kind of process was made and this is the reason why a deeper explanation is going to be made here to explain it under a deeper way, It is important to point out that the exhibition of complex mathematical expressions will not be exposed here but, on the other hand, an easy explanation of the process so that it can be easily assimilated by the reader although his/her knowledge is not very high.

2. Considerations about the process design.

The activated sludge process is usually carried out by introducing an organic residue inside a reactor in which two types of reactions take place:

(a) Oxidation and synthesis, In the first place, where, from the organic matter in the presence of oxygen and nutrients leads to its decomposition into carbon dioxide, ammonia and other finished products. All this reaction takes place in a reactor, which can be complete mixing or flow on piston, which entered the bacterial culture,

A1) Continous Stirred Tank Reactor with recirculation- As usual in this type of reactor, the degree of mixing is absolute and takes place with the following particularities:

The stabilization of waste takes place only in the interior of the reactor and the volume used to calculate the cell retention time only includes the reactor volume.

A2) Piston Flux Reactor with recirculation- The feature that defines this reactor is defined by a flow piston model where the particles that enter at the same time remain in the interior of the reactor the same period of time.

(b) The endogenous breathing. This second reaction causes the formation of end products along with the release of energy. In nature, the key role of bacteria is to decompose the organic matter of the incoming stream. the bacteria involved in this process are fundamentally Pseudomonas and nitrifying bacteria such as Nitrosoma and Nitrobacter. Bacteria not only break down the organic residue immediately, but they are adequate Floc. However, if the facilities are poorly designed, effluent may have a high content of bio-solids so the unit of secondary sedimentation must be redesigned or the aeration devices must be highly observed until the amount of incoming oxygen is adequate.

3. Other variables to be considered.

There are other series of variables that must be checked out in order to make sure that the active sludge process works correctly. Although some of them have already been discussed throughout the series of articles written on sewage, among these variables are:

3.a) The required quality of the effluent.

Requiring quality water output, the authorities will determine both the functioning of the process and the control of it. If required a high degree of treatment, the process must be highly controlled and probably it requires additional treatment. This quality should be determined through the analyses carried out in the laboratory.

The organic matter contents within the effluent is usually composed of the following differentiation:

3.a) 1.-Biodegradable soluble organic matter.

Matter not removed in the biological process come together compounds derived from the Lysis or cell death.

3.a) 2.-Organic matter in suspension.

They are part of this group not only colloids of the tributary that escape the treatment and separation.

3.a.) 3.-Non-biodegradable organic matter.

Both biological decomposition by-products and those initially present in the effluent belong to this group.

3.b.) Characteristics of the wastewater to be treated:

Flow rates and characteristics of the tributary, are outside the field of action of the operator, laboratory and competence of municipal authorities who control waste that poured into the collection system, avoiding that particular industries dumping toxic waste for the micro-organisms that work in the aeration tank.

3 c.) Number of active microorganisms that are needed in the treatment.

The ratio between the number of active microorganisms and available food, is a decisive parameter in controlling the process. If this proportion is not balanced, then some serious problems on the ground can appear. The number of organisms increases also increase the load of organic matter (food) and the time spent in the cuba of aeration (age of mud). The operator must remove excess of microorganisms (sludge in excess or purge of sludge) to maintain the optimal number of workers for the effective treatment of waters.

On the other hand, it is essential to proceed with a recirculation of sludge from the decanter to the aeration tank, in order to keep a sufficient concentration of organisms, already if not you go deleting and would end with a wash tank.

The decanter sludge must be removed as soon as the sludge blanket is formed since, remain in it, there may be events that make the mud to float. Sludge pumping system, therefore, must be to act when you need him in top condition.

To know the concentration of microorganisms in slurry and sludge recirculation of liquor, is determined by the level of volatile solids.

3 d) Dissolved oxygen level.

Oxygen which contributes to the aeration tank should be sufficient so that micro-organisms can't breathe and it can oxidize the organic matter.

The amount of oxygen amount of food relationship should be regulated and remain stable. A decompensation in one sense or another, can give rise to an appearance of filamentous organisms that tend to float in the secondary decanter, completely altering the solid-liquid separation and tending to be washed with the effluent.

The level of dissolved oxygen is often measured with sensors that give immediate oxygen quantities information in cuba, based on this information the agitation and aeration systems are put in place or are stopped.

Agitation should be well controlled, so the oxygen and food are homogeneously distributed throughout the vessel.

3 e) Retention time.

In order to produce biological oxidation process, it is necessary that microorganisms will remain a sufficient time of contact with sewage. This retention time is one of the parameters that must be taken into account to design the vessels because of that, in relation to the flow rate to try and time that must remain the flow in vessel, will decide the volume of it.

3 f) Sludge Volume Index.

It is defined as the volume in mL occupied by one gram of solids in suspension of mixed liquor, after sedimentation of 30 minutes into a test tube of 1000 mL. Therefore, we take 1 litre of liquor mixture and it starts to settle for 30 minutes, pointing the volume covering the mud and finding the relationship:

Sludge Volume Index (SVI) is an extremely useful measured parameter in a wastewater treatment process. It takes into account the 30-minute settleability test result and the activated sludge mixed liquor suspended solids (MLSS) test result to come up with a number that describes the ability of the sludge to settle and compact. SVI gives a more accurate picture of the sludge settling characteristics than settleability or MLSS alone.

SVI can indicate changes occurring in the activated sludge treatment process. By trending SVI data over a period of time, operators are able to prevent problems. Many textbooks give guideline SVI numbers, but since every plant operates differently, the best SVI for each plant will be different.

It is important to allow the sludge to settle in a quiet area letting sample should also be kept out of direct sunlight and refrain from using tall graduated cylinders for the settling test, as the friction created by the close walls can slow the settling, change settling velocities and give false readings. Wide-mouth containers that hold at least 1 liter are acceptable, but 2-liter containers are preferred.

How to interpret results

Here are some general guidelines for reading SVI:

SVI = 80 mL/g or less. This figure usually indicates a sludge that is dense and has rapid settling characteristics. This is most often attributed to an old, over-oxidized sludge typically seen in an extended aeration facility. The floc particles are dense and granular in appearance. As this type of sludge settles, it may leave a cloudy appearance in the supernate above the settled sludge blanket. This turbidity is called pinpoint floc (pin-floc). The sludge usually begins settling quickly after the start of the sludge settleability test, and it does not form larger particles before settling. Effluent BOD results may be below requirements, but TSS levels can still be high.

SVI = 100 to 200 mL/g. Most activated sludge plants seem to produce a clear, good-quality effluent with an SVI in this range. The sludge typically settles more slowly and traps more particulate matter as it forms a uniform blanket before settling. Microscopic examination of this MLSS would show an irregularly shaped floc particle with some filaments forming a backbone for floc-forming bacteria to attach and colonize.

SVI = 250 mL/g or higher. At this elevated SVI, the sludge settles very slowly and compacts poorly in the settleability test. The MLSS looks light and fluffy, not very dense. There are several reasons the SVI may be high.

During the startup, the sludge age is considered young and the floc particles are just forming. The MLSS result is usually low (less than 1,000 mg/L), and the supernatant above the sludge blanket will be cloudy, sometimes grayish/ green. This type of sludge usually leaves behind straggler floc particles that either settle slowly or not at all. Effluent BOD and TSS may still be above regulatory requirements.

A high SVI may also indicate filamentous sludge bulking. In this case, a microscopic exam might show light floc particles that contain long filaments extending out of the particle and touching filaments from other particles. The sludge can sit in the settleability test container for long periods and settle very little, or not at all.

Within the settleability test, the sludge first forms a blanket and seems to flocculate together before starting to settle. This usually happens in the first five minutes of the test. As the particles come together, they form larger particles that have a specific gravity greater than water. As the sludge settles, channels through the sludge that are formed by the liquid being squeezed out of the sludge as it compacts will be noticed.This value gives the behavior of sludge in the decanter. If the value is less than 100, it means that sludge with development agencies which settle well and therefore good solid-liquid separation. If the value is greater, filamentous organisms with bad sedimentation, have been developed which leads to an unbalanced event in the system functioning. This unbalance is known as "bulking" and was already explained in an article published on May 13, 2015.

3. g) Nutrients requirements.

So that a biological system works properly is necessary the presence of nutrients in an order of approximately 12.5% of nitrogen, while phosphorus needs are around 20% of this value. It is important to know the fact that nutrient requirements are smaller for processes with greater cell retention time.

4.- Final conclusions.

Throughout this article, some important aspects of the activated sludge process have been tried to be exposed. First of all, a slight explanation of the process have been done and they became deeper when the exposition was bein explained. I hope the explanations of the technical terms of it have been clearly exposed.

Thanks very much for reading this article as it encourages me to improve the way I am writing these reports.