Your job is to design a wastewater treatment plant based on the information provided.
Design packet should include
1) a process schematic
2) a table of mass balances at average flow
3) layout of the plant in the area
4) calculations for the design of every unit operations and processes,
5) justifications (or assumptions) for the calculations.
You can choose or combine any unit operations and/or processes that you have learned in this class if the system meets the effluent requirement.
For the separate treatment of primary and secondary sludges
- Gravity thickening of primary sludge Air floatation thickening of excess activated sludge Anaerobic digestion Chemical conditioning Vacuum filtration or centrifugation Landfill disposal
- Gravity thickening of primary sludge Air-floatation thickening of excess activated sludge Anaerobic digestion Chemical conditioning Vacuum filtration or centrifugation Multiple-hearth or fluidized-bed incineration Landfill disposal
Thickening refers to the procedures that are used to increase the solid content of the sludge through the removal of the liquid fraction. The process of thickening is usually accomplished through physical means which include the co-settling, floatation, centrifugation, gravity settling, rotary drum and gravity belt (Ainsworth, 2016). The volume reduction which is obtained by the sludge concentration is very useful to the subsequent treatments process, such as digestion, drying ,dewatering and combustion from the following standpoints
- Quantity of the chemical that are needed for the sludge conditioning
- The capacity of the tank and the equipment that are required
- The amount of heat needed by the digester and the amount of the auxiliary fuel need for the heat drying or the incineration
The table below gives the most common used methods of thickening in sludge process.
Table 1: Thickening methods used in the sludge process
The sludge thickener location within the wastewater treatment plant is very important. In the situation where sludge is intended to be thickened a mix of the waste activated sludge and primary sludge is an appropriate culture. If the resulting sludge’s are intended to be dewatered, they should be separately thickened and then blended instantly before the dewatering process is carried out.
Activities of a sludge thickener
This sludge thickener usually operates with the same principle of the settling tank. The sludge enters the thickener from the middle and then it is distributed radially, they are then forced to settle and then collected from the underflow, the effluent usually exists over the weirs.
In the thickness which operate for long. There are difference areas of concentration. In most cases the topmost zone is solid free and it usually comprises the liquids which are later removed over the weirs.
In a thickener which is continuously operated, there are various zones of concentration. Usually the topmost dear zone is free of the solids and it usually comprises the liquids that are eventually escaped over the weirs. The next zone is usually referred to as the feed zone .In this zone it is characterized by a uniform solid concentration .Below the feed zone is a zone of the increasing solid concentration. This zone is referred to as compaction zone.
The sludge blanket is defined as the top of the feed zone ,The thickness of the blanket is the key operational control that the treatment plant operator has over the thickener. Through the increase of the underflow rate , the operator can lower the blanket , and hence the solids residence time is also lowered, throughput of the solids is increased and the solid concentration in the underflow is decreased. The operator then would have a higher reserve volume in case there is unexpected heavy sludge load coming. In the situations where the sludge blanket is high it will lead to the underflow solid concentration to be high because of the high solid residence time (Turovskiy, 2017). One challenge with this approach is the formation of gas due to the anaerobic activity. The gas formed will cause the floatation of the solids in the thickener. The chemicals such as chlorine need to be added in order to inhibit the biological activity .A well operated thickener will have a solids recovery of approximately 95%.
Design of thickeners
Thickening Methods Used in the Sludge Process
The thickening process usually takes place in the settling tank with a long-enough solid retention time. Actually the thickening o0f the sludge is of great concern to the operator where he desires a high underflow solids concentration. So is the general practice to design these process in order to achieve better performance and clarification, There are two techniques of designing the thickeners which include;
- Design based on the experience
- Design based on the laboratory data
Solids via output is very crucial criterion in the design of thickeners. The design is usually based on the solids flux. That is kg solids/h/m2 .Typical flux values are illustrated below.
The design by the use of this approach involves the selecting of a typical solid flux and then calculating the required surface area through the division with the anticipated solid feed by the flux (Brandt, 2010).
Are of the thickener is usually obtained through the formula below
Design based on the laboratory data
This is the best method of design thickeners if the laboratory data is available. A typical test is usually carried out by using a 1000ml-graduated cylinder. Then the sludge is mixed homogenously and let to settle in the cylinder. In a few moments an interface which is aimed at separating the solids and the clear water at the top is carried with a given settling velocity (Association, 2013). This velocity of the interface is determined with respect to the time. Interface height is then plotted against the zone settling velocity which is calculated from the initial slope of the graph. The graph is shown in the figure below.
Fig 1: Interface during settling test.
Sludge dewatering refers to the separation of liquids and solids whereby, generally, the least possible residue moisture is required in the solid phase and the lowest possible particles are needed in the separated liquid phase. A good example of this process is the dewatering of sludge from the municipal sewage plant or the treatment of industrial waste waters (Jeffrey J. Peirce, 2015).
The process of dewatering usually lowers the weight and volume of the solid wastes, thus reducing costs such as landfill and transport at the same time it enhances its suitability for the subsequent utilization. The process stabilizes wastes while reducing leachates and offers a more uniform final product. The performance from the mechanical dewatering n is very critical to downstream, energy recovery from the combustion .While the polymer use can improve dewatering the process is expensive and can be challenging downstream.
The option of mechanical dewatering usually dewater sludge to 30 to 50 percent solid content depending on the combustion and the ability to retain sludge .Nevertheless with the paper and pulp sludge solid contents of 20-30 % are mostly achieved.
The primary solids in the sludge are made up of fines, fibers and they are simple to mechanically dewater. The secondary sludge from the biological and the chemical treatment ids very complex to dewater mechanically and the secondary solids are commonly combined together with the primary sludge to enhance its dewatering properties.
Chemical additives such as acids, inorganic conditioning compounds and surfactants are applied to enhance flocculation properties and the dewatering. Thermal conditioning of the biological b sludge by heating is believed to enhance dewatering to 30-40 % b solids as opposed to the up 20% solids for the chemical conditioning (Moore, 2010).
The sludge is normally conditioned prior to its thickening and dewatering. In most cases two types of condition chemicals are applied to improve the dewatering of sludge and they are;
- Mineral chemicals such as lime and iron salt. The chemicals are often found in the filter press applications
- The organic chemicals such as coagulants and the flocculants .The most common type of the flocculants are usually encountered in nature.
In this method the centrifuges dewaters continuously with the aid of centrifugal forces of many thousand gs.
The principle of a centrifuge is also referred to as centrifugal decantor , this principle is to use the centrifugal force in order to accelerate the solid-liquid separation. For the purpose of simplifying it can be assumed that a centrifuge is a conical cylinder decantor which turns horizontally on its own axis with a clarified water flow , the dewatered sludge which is being removed by an Archimedean screw (Bajpai, 2014). The rotation applies a centrifugal force on the solid particles which then rotates at very high speeds.
In real situations the flocculated sludge is injected inside the centrifuge bowl via the injection pipe. The bowl usually rotates at very high speeds of approximately 350rmp and the particles are flattened against the walls of the bowl in the clarification zone. The particles are then moved by the Archimedean screw towards the end of the bowl’s cone in the sludge spin-dry zone .The clarified liquid is referred to as centrate is removed at the other end of the bowl by the overflow. The figure below show the centrifuge.
Fig 2 centrifuge.
Ainsworth, B. (2016). Handbook Biological Waste Water Treatment - Design and Optimisation of Activated Sludge Systems. London: Webshop Wastewater Handbook.
Association, A. W. (2013). Water Treatment. Sidney : American Water Works Association.
Bajpai, P. (2014). Management of Pulp and Paper Mill Waste. Berlin: Springer.
Brandt, M. J. (2010). Water Supply. Chicago: Malcolm J. Brand.
Gregorio, D. D. (2016). Chemical Primary Sludge Thickening and Dewatering, Volume 1. London: Municipal Environmental Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, .
Jeffrey J. Peirce. (2015). Environmental Engineering. Los Angeles : Butterworth-Heinemann.
Moore, B. A. (2010). Process control and troubleshooting in municipal sludge thickening. New York : University of Wisconsin--Madison,.
Turovskiy, I. S. (2017). Wastewater Sludge Processing. Texas: John Wiley & Sons.
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