Chapter 2 -of 10 Pretreatment PDF

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WWST - Chapter 2

WS 20/21

Chapter 2: Pre-treatment This chapter gives an overview of the components of the preliminary treatment process at a typical wastewater treatment plant

Objectives and competences After completion, the student should be able to:       

Explain the purpose and components of preliminary treatment, and how and where they fit into the overall wastewater treatment process Identify the various types of screening equipment, their functions, and the proper method of disposing of the material that is collected Describe and discuss the principles of operation of the various types of grit removal systems Understand the purpose of grit removal Describe the major components of septage-receiving stations and how they function Identify potential odour sources in a preliminary treatment system and alternatives for controlling the odours Discuss the common methods of flow measurement and flow management

Content Chapter 2: Pre-treatment ....................................................................................................... 1 Objectives and competences ............................................................................................. 1 2.1

General information ................................................................................................. 3

2.2

Septage/Faecal sludge receiving............................................................................. 3

2.2.1 2.3

Septage receiving station ................................................................................. 4

Screens and sieves ................................................................................................. 5

2.3.1

Bar screens (Coarse screens) .......................................................................... 7

2.3.2

Fine screens ...................................................................................................11

2.3.3

Sieves .............................................................................................................15

2.3.4

Screens costs..................................................................................................16

2.3.5

Special applications of screens .......................................................................16

2.3.6

Typical problems with screens ........................................................................17

2.3.7

Screening handling .........................................................................................17

2.4

Coarse solids reduction ..........................................................................................19

2.5

Grit removal............................................................................................................20

2.5.1

Types of Grit chambers ...................................................................................21 1

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2.5.2

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Grit removal and management ........................................................................25

2.6

Fat and grease removal .........................................................................................27

2.7

Flow management..................................................................................................27

2.7.1

Parshal flume ..................................................................................................28

2.7.2

Flow equalization ............................................................................................29

2.8

References.............................................................................................................29

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2.1 General information In the preliminary treatment (or pre-treatment) the main objective is the removal of the debris (sticks, rocks, sand, grit, gravel, bottles, rags, plastic parts and similar items) from the receiving wastewater, separating them from the other bio-solids (wastewater sludge) generated in the WWTP. Objects are removed at this stage to protect pumps and other equipment downstream of the plant from clogs, jams or excessive wear. The preliminary treatment components depend on:   

Collection system: must be more intense in combined sewers Wastewater sources: bringing different materials, and Age and size of the WWTP: the technologies have evolved in the last decades and different technologies are suitable form different plant sizes.

The preliminary treatment consists usually on the removal of coarse solids by screens, the removal of sand and grease in a grit chamber and ends in the flow measurement.

Figure 1: Simplified schematics of typical preliminary treatment processes (Sperling 2007)

2.2 Septage/Faecal sludge receiving Septage is a general term for the contents removed from septic tanks, portable vault toilets, privy vaults, holding tanks, very small wastewater treatment plants (WWTP), or semi-public facilities (i.e., schools, motels, mobile home parks, campgrounds, small commercial endeavours) receiving wastewater from domestic sources. Septage is partially digested anaerobic wastewater and compared to raw domestic wastewater from a conventional municipal sewer collection system, septage usually is quite high in organics, grease, hair, stringy material, scum, grit, solids, and other extraneous debris. Substantial quantities of phosphorus, ammonia nitrogen, bacterial growth inhibitors, and cleaning materials may be present in septage depending on the source.

Insertion: Septic tank A septic tank is a system for the pre-treatment of household wastewater (wastewater from toilets, showers or tubs, and kitchen or laundry sinks), when no sewer is available.

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A septic tank consists on an underground chamber made usually of concrete, fiberglass or plastic, with typically two or more tanks in series, designed for the removal of larger solids and particulated organic material. The tank of the septic system lets heavy solid materials settle to the bottom, while the lighter wastewater stays at the top. This liquid then flows out of the tank into underground perforated pipes, called leaching bed or septic drain field, where it filters into the ground and is further treated by bacteria and other soil organisms. The sludge in the septic tanks has to be removed periodically and is called septage or faecal sludge.

Figure 2: Septic tank with 3 closed tanks connected in series (Source: Gujer [2002])

2.2.1 Septage receiving station Some WWTP accept septage from tanker vehicles who pump out septic tanks, and similar. The septage receiving stations are usually placed at the beginning of the treatment plant and dosed to the pre-treatment stage. The flow of the septage has to be carefully monitored and managed and the septage has to be stored and pumped slowly in to the plant. Some septage receiving stations are modular equipments, include a series of pre-treatment steps, including screens, screenings dewatering, grit chamber, grease trap and sand washer

Figure 3: Schematic drawing of a septage receiving station (WEF, 1994)

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2.3

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Screens and sieves

The wastewater stream contains numerous coarse materials that could cause problems or damage in downstream plant sections (e.g. clogging, clogging, silting). In the screens, coarse materials are removed from the wastewater in order to avoid operational problems. Typical coarse materials are:    

Hygiene products (e.g. toilet paper, condoms, sanitary napkins, cotton swabs, etc.) Plaiting materials (e.g. Hair, fibers) Coarser wastewater components (e.g. food waste) Impurities (e.g. stones)

Note: Wet toilet paper has lead to several problems in the sewer systems and WWTP in the last few years. Problems can appear already in the pipelines in houses and buildings! If the raking system does not work well, plaiting materials can also impair the efficiency of the aeration system (see Figure 5).

Figure 4: Submersible pump not functional because of coarse materials (left); coarse materials after removal (right) (Source: AV Freigericht)

Figure 5: Aeration system in an activated sludge tank, blocked with plaiting materials (Source: GKU)

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Figure 6: Mixing system of an anaerobic reactor for sludge digestion blocked with hair and other coarse materials (Source: Stadtwerke Halle)

Note: If the screening system is not working well, screenings can even impair sludge digestion! The motors of the mixing systems are overloaded or not able to function at all.

In order to avoid flooding in case of a malfunction of the screen, an emergency bypass usually ensures that the wastewater is discharged without damage, so the wastewater continues its treatment. Bar racks and screens remove larger inorganic debris /rags, cans etc. prior to downstream plant processes. The separated screenings are washed to remove the organics contained in the wastewater, since they are useful for further processes in the plant treatment (i.e. denitrification). After washing and dewatering, the screenings are usually landfilled or incinerated (depending on the country). Depending on the system, sometimes two stages (coarse followed by fine) screens are used. Due to the wastewater characteristics, combined sewer systems require heavier screening units as separate systems, even already in the sewer system. In the following table, a classification of (some of) the existing types of screens is presented. Different types of screens can be classified due to the operation method: co-current, counter current, belt filter, drums, etc.

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Table 1: Overview of screen/sieve types (modified from Metcalf & Eddy) Category

Screen opening

Screen type Bar rack Chain driven Reciprocating rake Mechanically cleaned Catenary Continuous belt Static wedge wire Internally fed Drum Externally fed Step Continuous belt (other) Spiral sieve Fine sieve Manually cleaned

Coarse screen (Bar racks)

50 to 150 mm

Fine screen

< 6 mm

Sieve

< 2 mm

2.3.1 Bar screens (Coarse screens) Coarse screens are most often based on bar screens. At older sewage treatment plants in Germany and in other countries, coarse screens are still frequently found. At smaller WWTP they are sometimes even hand-cleaned. They are also used in emergency bypasses or for simple faeces-sludge collection stations. In case of only a few deliveries of faecal sludge, this solution may still be acceptable in order to avoid operational problems; alternatively, it can be dosed to the inlet of the treatment plant. However, for frequent deliveries, it is recommended to install a separate pretreatment system for the faecal sludge (e.g. comminutors, macerators, later in this chapter).

Figure 7: Hand-cleaned bar coarse screen at a simple faeces collection station

2.3.1.1 Manually-cleaned bar screens These type of screens are frequently used ahead of pumps in small wastewater pumping stations and sometimes at the headworks of small to medium-size WWTP. Often they are used for standby screening in bypass channels for service during high flow periods, when mechanically cleaned screens are being repaired or in case of power failure (Metcalf &Eddy).

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(a)

(b) Figure 8: Hand-cleaned screens: (a) and (b) in an emergency overflow; (c) as pre-treatment stage for industrial wastewater (Source: Wiese)

(c)

Figure 9: Cut-away sketch of a manually cleaned bar screen (Source: WEF, 1996)

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2.3.1.2 Mechanically cleaned bar screens The design of mechanically cleaned bar screens has evolved over the years to reduce maintenance and improve the screenings removal capabilities. There are four principal types: 1. Reciprocating rake (climber screen) The also called climber screen, imitates the movements of a person raking the screen. The rake moves to the base of the screen, engages the bars, and pulls the screenings to the top of the screen where they are removed. The main advantage is that there are no submerged moving parts, which makes maintenance easier. The main disadvantages are that the cleaning cycles are longer and there might be some accumulation of screenings at the bottom, impeding the rake movement.

Figure 10: Reciprocating rake (Source: PassavantGeiger)

2. Chain-driven screen These type of screens can be divided in upstream cleaned (cleaned from the front), or downstream (cleaned from the side or back). The main advantage is that there are multiple cleaning elements (i.e. not only the rake, but also e.g. perforated plates, etc.). This helps to achieve shorter cleaning cycles. The main disadvantage is that there are submerged mechanical/ moving parts, meaning that the well must be emptied for maintenance purposes.

Figure 11: Chain-driven screen (Source: Metcalf & Eddy)

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3. Catenary screen Catenary screens are front cleaned-front return chain driven screen, without submerged sprockets. The main advantage is the lower jamming risk. A disadvantage is the large footprint due to the angle of inclination of the screen (45 to 75°).

Figure 12: Catenary screen (Source: Metcalf & Eddy, Fairfield)

4. Continuous belt This is a continuous, self-cleaning screening belt that removes fine and coarse solids. A larger number of screening elements (rakes, teeth, combs, hooks, steps) are attached to the drive chain. The screen has no submerged sprocket, which means that most maintenance can be done above operating floor. Moreover, depending on the screen openings, the unit is difficult to jam. Since many elements compose the screen, overhaul or replacement of the screening elements is a time-consuming and expensive operation. Figure 13: Continuous belt screen (Source: Metcalf & Eddy) 10 Vergara – OVGU

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Table 2: Typical design information for manually and mechanically cleaned bar screens (Metcalf & Eddy, 2003)

Parameter Width Depth Clear spacing between bars Slope from vertical Approach Maximum velocity Minimum Bar size

Cleaning method Manual Mechanical 5 – 15 5 – 15 25 – 38 25 – 38 25 – 50 15 – 75 30 – 45 0 – 30 0.3 – 0.6 0.6 – 1.0 0.3 – 0.5

Units mm mm mm ° m/s m/s

2.3.2 Fine screens Fine screens have as function to remove fine contaminants, debris, plastics that are not removed on coarse rack screens. They can be used not only for preliminary treatment (Pretreatment) but also as primary treatment1 in e.g. trickling filters. Fine screens retains all solids larger than grit out of influent and also trap a high percentage of organic solids. The retained material has to be washed to prevent odours and to return to the influent organic carbon sources, which are useful in later stages of the process. The wash water has to be returned to the biological treatment. The minimum bar space can be 2 mm. There are different types of fine screens: 2.3.2.1 Static wedge wire screen The wedge wire consists of small, stainless steel wedge-shaped bars with the flat part of the wedge facing the flow. A considerable floor area is required and the screens must be cleaned daily with high pressure hot water or degreaser to remove grease build up. Applicable for small scale plants (Metcalf & Eddy) .

1

Note: Primary treatment will be seen in detail in Chapter 3, Clarifiers. 11

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Figure 14: Static wedge wire screen (Source: Köser, Metcalf & Eddy); Wedge wire (Source: Multimesh.com)

2.3.2.2 Rotary drum screen The screening medium is mounted on a cylinder that rotates in a flow channel. The sizes are from 0.9 to 2 m diameter and from 1.2 to 4 m length. Rotary drum screens can be: 



Internally fed: the wastewater flows into one end of the drum and outward through the screen with the solids collection on the interior surface. Applicable for flowrates between 0.03 – 0.8 m3/s Externally fed: the wastewater flows on top of the unit and passes though the interior with solids collection on the exterior. Applicable for flowrates < 0.13 m3/s (Metcalf & Eddy).

Figure 15: Externally fed drum screen (source: Dontech Industries)

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Figure 16: Drum screen with external, rigid scraper on a brewery wastewater treatment plant (Source: Wiese)

2.3.2.3 Escalating step screen The design consists of two step-shaped steps of thin vertical plates, ones fixed, one movable. The fixed and the movable step plates alternate across the width of an open channel and together form a single screen face. The movable plates rotate in a vertical motion. Through this motion, solids captures on the screen face are automatically lifted up to the next fixed step landing, and are then eventually transported to the top of the screen, where they are discharged to a collection hopper (Metcalf & Eddy) . It is a filigree type of screen, so that it can be damaged by stones or similar. It must be checked if it is necessary the installation of an upstream coarse screen.

Figure 17: Step screen (Source: Wiese)

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Figure 18: Step screen, hook type (Source: Wiese)

2.3.2.4 Other continuous belt type screens Perforated plate screen

(b) Figure 19: (a) Perforated plate screen and rotating cleaning brush in the head section (Source: Wiese); (b) Perforated plate screen (Source: Huber SE)

(a)

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2.3.3 Sieves Sieves are able to retain smaller materials, and therefore can be used either for pre-treatment of for primary treatment. Such systems are especially used when all coarse impurities have to be removed from the wastewater in order not to endanger the following special technology (e.g. membrane) For about 20 years, spiral screens have been used frequently, especially in small and mediumsized sewage treatment plants. The usual sizes are from 1 to 6 mm, the design is compact and they are relatively insensitive to clogging/blocking. Nowadays, it is possible to find spiral screens with very small openings (...