feasibility study of salsnes filter™ fine mesh sieves...

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Date: 07.12.00 : 17 Report No: 00-054 Version: 1

Feasibility study of SalsnesFilter™ fine mesh sieves used for

primary treatment atmunicipal wastewater

treatment plants

Aquateam - Norwegian Water Technology Centre A/S

Report no: 00-054Project no: O-00154

Project manager: Dr. Bjørn RustenCo-worker: Bjarne Paulsrud, M.Sc.

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aquateam REPORTP.O. Box 6875 Rodeløkka Report number: 00-0540504 Oslo Norway Availability: OpenTelephone: +47 22 04 12 40Telefax: +47 22 04 12 10

Date

December 7, 2000

Title

Feasibility study of Salsnes Filter™ fine mesh sievesused for primary treatment at municipal wastewatertreatment plants

Number of pages

16Responsible sign.

Bjarne Paulsrud

Author(s) sign.

Bjørn Rusten

Bjarne Paulsrud Project number

O-00154

Client(s)

Salsnes Filter AS

Contact person(s)

Ivar Solvi

Summary The feasibility of using Salsnes Filter fine mesh sieves (with 300 mm mesh size) instead of con-ventional sedimentation for primary treatment of municipal wastewater, was evaluated for a generictreatment plant with an average flow of 18000 m3/d. Flow sheets for 4 process combinations werepresented, covering primary treatment only, and primary treatment followed by a nitrifying activatedsludge plant. The different unit processes and reactors were designed and dimensioned based ongiven criteria, and finally the total cost and area requirement for a process, or the difference in costand area requirement between processes, were calculated for each alternative. The cost compari-sons were made for outdoor plants with open basins. The cost for land was set at zero. For primary treatment only, the Salsnes Filter fine mesh sieves needed less than 10 % of thearea required for conventional sedimentation, and the investment cost was about 50 % of the cost forconventional sedimentation. For primary treatment including thickening and dewatering, the SalsnesFilter fine mesh sieves were also expected to save NOK 100 000 per year in O & M costs. For plantswithout thickening and dewatering, the O & M costs were somewhat lower for sedimentation than forthe Salsnes Filter fine mesh sieves. The reason for this is that dewatering is an integral part of thesieves, and that the energy consumption for operation of the sieves is higher than the energy con-sumption necessary for the intermittent operation of sludge pumps and scrapers in a conventionalsedimentation process. For primary treatment followed by nitrifying activated sludge, use of Salsnes Filter fine meshsieves will save about NOK 3.5 million in investment . Due to the large savings in investment costs, application of Salsnes Filter fine mesh sieves willresult in significantly reduced annual costs (capital + O & M) for all alternatives considered. With acost on land and/or use of covered reactors, application of Salsnes Filter fine mesh sieves will resultin even larger savings.

Key words - English Key words - NorwegianFine mesh sieve Finmasket silPrimary treatment Mekanisk resningDewatering AvvanningSedimentation SedimenteringCost Kostnader

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Table of contents

1. Introduction ......................................................................................................................... 52. Description of the Salsnes Filter fine mesh sieves............................................................. 63. Design examples ................................................................................................................ 8

3.1. Wastewater flows and composition............................................................................. 83.2. Expected primary treatment performance................................................................. 103.3. Design values ............................................................................................................ 103.4. Design of processes for primary treatment............................................................... 11

3.4.1. Alternative A........................................................................................................... 113.4.2. Alternative B........................................................................................................... 12

3.5. Design of processes for primary and secondary treatment...................................... 133.5.1. Alternative C........................................................................................................... 133.5.2. Alternative D........................................................................................................... 13

4. Comparison of alternatives............................................................................................... 154.1. Cost and area requirement........................................................................................ 154.2. Discussion.................................................................................................................. 16

5. References........................................................................................................................ 17

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1. IntroductionSalsnes Filter, with head office in Namsos, Norway, has developed a unique fine mesh sieve sys-tem for treatment of municipal and industrial wastewater. The first prototype was produced in 1992.Presently, more than 20 systems have been installed for treatment of municipal wastewater.

Traditionally fine mesh screens and sieves have been used for pretreatment, or as the only treat-ment at smaller plants that discharged wastewater to the ocean. Removal efficiencies reported forscreens and sieves with less than 1 mm openings, were only 5 to 10 % for organic matter and only10 to 20 % for suspended solids (SS) (Hansen, 1996). The effect was mainly aesthetic, as thesieves and screens only removed the most visible part of the pollution.

The European Union has defined primary treatment as removal of at least 50 % SS and at least 20% BOD5. This can normally not be achieved by just fine mesh sieves or screens. However,Salsnes Filter has achieved primary treatment performance at their installations. In addition to re-moving particulate material, the Salsnes Filter unit also dewaters the sludge, resulting in a verycompact process for primary treatment. A recent test at the Tiendeholmen wastewater treatmentplant (Rusten, 2000) showed that two Salsnes Filter fine mesh sieves in parallel (with mesh sizes of300 mm and 350 mm) removed 59 % of the SS, 45 % of the COD and 36 % of the BOD5. The pri-mary sludge leaving the sieves had an average total solids (TS) concentration of 23 %.

This report will look at the feasibility of using Salsnes Filter fine mesh sieves instead of conven-tional sedimentation for primary treatment at municipal wastewater treatment plants. One scenariowill be for primary treatment only. The other scenario will be for primary treatment followed by anitrifying activated sludge plant.

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2. Description of the Salsnes Filter fine mesh sievesThe Salsnes Filter fine mesh sieves are compact units for mechanical separation of particulatematerials from wastewater. Currently, models with capacities from 10 to 180 liters per second (36to 648 m3/h) and mesh sizes from 0.1 to 1.0 mm are available.

Sketches of a Salsnes Filter sieve unit are shown in Figures 1 a and b. The wastewater flowsthrough the inlet tube (1), filters through an endless wire cloth and 40 to 80 % of the SS is removed.From the back of the wire cloth filtered water flows out through the outlet tube (3). The wire clothrotates as the arrows show. The cloth transports the separated SS (sludge) to the air cleaning de-vice (9) where compressed air blows the sludge down into the sludge compartment (8). First stageof dewatering is done by gravity during transport to the screw compartment. The screw presses thesludge forward to a press cylinder (18) where further dewatering is done. The dry solids concen-tration of the sludge can be regulated by adjusting the tension on the spring-loaded lid (20).

The wire mesh cloth may be produced using different types of plastic, different thickness of thethread and different mesh sizes. For maintenance of the wire mesh cloth, it is flushed (11) with hotwater twice a day (2 x 20 liters) to remove fat and grease.

A pressure transmitter (4) measures the level of incoming water and this information is used to varythe speed of the wire mesh cloth to achieve optimum performance at variable flow rates and vari-able influent SS concentrations. As long as the water level in the inlet chamber of the sieve is low,the wire mesh cloth is immobile. Eventually particles will accumulate on the cloth surface, the wa-ter level will increase and the pressure transmitter (4) will automatically start the motor (16) thatmoves the wire mesh cloth. If the water level keeps increasing while the cloth is moving, the speedwill automatically increase. If the water level drops below a preset limit, the motor will stop until thelevel increases again.

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a) Cut from the right side

b) Cut from behind

Figure 1. The Salsnes Filter fine mesh sieve.Key:1 Inlet 9 Air cleaning device 16 Gear/motor for wire cloth2 Overflow 10 Rubber scraper 17 Hot water nozzles for3 Outlet 11 Hot water nozzles cleaning press cylinder4 Level indicator 12 Screw 18 Press cylinder5 Wire cloth 13 Cold water pipe for settled 19 Reject from press cylinder6 Wastewater waste removal 20 Spring-loaded lid7 Filtered water 14 Drain valve for settled waste 21 ventilation8 Sludge compartment 15 Gear/motor for screw press

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3. Design examplesFigure 2 shows simplified box diagrams of the treatment processes that will be evaluated. Sidestreams and mixing tanks and storage tanks for sludge are not shown. Processes A) and B) are forprimary treatment only. Processes C) and D) are for primary treatment followed by a nitrifying acti-vated sludge plant.

Process A) is primary treatment with Salsnes filter fine mesh sieves and the same pre-treatment asused at Tiendeholmen wastewater treatment plant in Namsos. Sand and grit traps are not neces-sary to achieve good removal efficiencies. They have been included because they significantlyreduce the wear of the sieves and prolong the lifetime of the wire cloth. The primary sludge is de-watered to the desired solids content, for example 8 % total solids (TS) if followed by anaerobicdigestion, or 25 % TS if followed by further treatment and/or disposal methods that require highsolids concentrations.

Process B) is traditional primary treatment, with pre-treatment and primary sedimentation. Thick-ening of the primary sludge is recommended if followed by anaerobic digestion. For further sludgetreatment and/or disposal methods that require high solids concentrations, both thickening and de-watering will be necessary.

Process C) is the same primary treatment as in process A), followed by a nitrifying activated sludgeaeration tank and secondary sedimentation. The mixture of dewatered (25 % TS) primary sludgeand thickened (2 % TS) secondary sludge goes to anaerobic digestion and finally to dewatering.

Process D) is the same traditional primary treatment as in process B), followed by a nitrifying acti-vated sludge aeration tank and secondary sedimentation. The mixture of primary sludge and sec-ondary sludge goes to a thickener, then to anaerobic digestion and finally to dewatering.

3.1. Wastewater flows and composition

The design example is done for a treatment plant with the following design flows:

• Average flow: Qavg = 18000 m3/d• Design dry weather flow: Qdesign = 1000 m3/h• Design wet weather flow: Qmax = 1500 m3/h

This results in a ratio of 1.33 between the design dry weather flow and the average flow, and a ratioof 2.0 between the design wet weather flow and the average flow. These are normal ratios for aplant of this size.

Influent wastewater has the following composition:

SS = 250 mg/LTotal COD = 360 mg/LFiltered COD = 100 mg/LBOD5 = 180 mg/LFiltered BOD5 = 60 mg/L

The influent wastewater composition is assumed to be a typical medium concentration wastewater,based on data obtained by Aquateam from several plants in different parts of the world.

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Coarsescreens

Sand &grit trap

Salsnes Filterfine mesh

sieves

Primarysedimentation

Thickening Dewatering

Dewateredprimary sludge

EffluentInfluentA)

B) Coarsescreens

Sand &grit trap

EffluentInfluent

Coarsescreens

Sand &grit trap

Salsnes Filterfine mesh

sieves

Primarysedimentation

Thickening

Dewateredprimary sludge

EffluentInfluent

C)

D) Coarsescreens

Sand &grit trap

Influent

Nitrifyingactivatedsludge

Secondarysedimentation

Thickening

Anaerobicdigester

Dewatering

EffluentNitrifyingactivatedsludge

Secondarysedimentation

Anaerobicdigester

Dewatering

Figure 2. Process alternatives using Salsnes Filter fine mesh sieves or primary sedi-mentation for primary treatment.

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3.2. Expected primary treatment performance

For primary treatment with Salsnes Filter fine mesh sieves, filter cloth with a mesh size of 300 mm ischosen. For the given wastewater composition, the removal efficiencies shown in Table 1 are ex-pected over the fine mesh sieves.

Removal efficiencies over primary sedimentation tanks will depend on the wastewater compositionand the surface area and water depth of the tanks. At plants where Aquateam has gathered data,removal efficiencies were from 41 to 55 % for SS, from 25 to 45 % for total COD and from 18 to 25% for total BOD5. The overflow rates at these plants were from 0.6 to 2.3 m3/m2/h. The Germandesign guidelines (ATV, 1991) use 50 % SS removal and 25 % BOD5 removal at hydraulic retentiontimes of 1 to 1.5 hours in primary sedimentation tanks. Based on the available information, removalefficiencies as shown in Table 1 have been chosen for sedimentation tanks with an overflow rate of2.0 m3/m2/h at design dry weather flow, and a water depth of at least 2.5 m.

Table 1. Expected primary treatment performance.

ParameterSalsnes Filter

fine mesh sieves,% removal

Primarysedimentation,

% removal

Suspended solids 60 50

Total COD 40 35

Total BOD5 30 25

3.3. Design values

The design values used for different processes are shown in Table 2. Only unit processes wherethe choice of primary treatment will influence either the need for the unit, the size of the unit, or theoperation cost of the unit, will be addressed. Coarse screens and sand traps, for example, will bethe same in all alternatives and will therefore not be considered when comparing process alterna-tives. The same goes for the secondary sedimentation tanks, that will be identical in process alter-natives C) and D).

The nitrifying activated sludge aeration tanks will be influenced by the primary treatment perform-ance. The need for thickening and the size of the anaerobic digesters will be influenced by thesolids concentration of the primary sludge leaving the primary treatment units.

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Table 2. Design values used in examples.

Salsnes Filter, model 5000/6000, 300 mm mesh size: 540 m3/h capacity

Primary sludge dewatered in Salsnes Filter unit: £ 25 % TS

Primary sedimentation tank overflow rate at design dryweather flow:

2.0 m3/m2/h

Primary sedimentation tank water depth: 3.0 m

Primary sludge from sedimentation tank: 3.5 % TS

Thickened primary sludge: 6 % TS

Biological sludge from secondary sedimentation tank: 1 % TS

Thickened secondary sludge: 2 % TS

Water depth in thickeners: 3.5 m

Solids load, thickening of primary sludge: 100 kg TS/m2/d

Solids load, thickening of secondary sludge: 25 kg TS/m2/d

Solids load, thickening of primary-secondary sludge: 50 kg TS/m2/d

Activated sludge aeration tank at 12 °C: 3000 mg/L MLSS, 8 d SRT

Activated sludge aeration tank water depth: 4.0 m

Activated sludge oxygen demand: As per ATV guidelines (ATV, 1991)

Biological sludge production: As per ATV guidelines (ATV, 1991)

Anaerobic sludge digestion: 15 d SRT at 8 % TS 12 d SRT at £ 6 % TS

3.4. Design of processes for primary treatment

3.4.1. Alternative A

For primary treatment with Salsnes Filter fine mesh sieves, three units of the model 5000/6000 willbe needed to accommodate the peak hour flow. Separate dewatering of primary sludge is not nec-essary, as the sieves can produce a sludge with up to 25 % TS. If the sludge goes to anaerobictreatment, the sieves can be adjusted to produce primary sludge with 8 % TS.

Key data are shown in Table 3. The process is very compact. Water treatment and primary sludgedewatering can be accomplished within an area of approximately 50 m2. With this area, there is 0.6m of free space outside the extremities of each machine on three sides, and 1.5 m of free space onthe fourth side. Area for sludge storage and sludge containers is not shown, since this will be aboutthe same for the different alternatives.

Table 3. Key data for primary treatment with Salsnes Filter fine mesh sieves.

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Primary treatment equipment: 3 x Salsnes Filter model 5000/6000

Total space requirement: ~ 50 m2

Primary effluent quality: 100 mg SS/L

216 mg COD/L

126 mg BOD5/L

Average primary sludge production: 2700 kg TS/d

Primary sludge volume at 25 % TS: 10.8 m3/d


Anaerobic digester volume (optional): 506 m3

3.4.2. Alternative B

For primary treatment in conventional sedimentation tanks, two parallel lines are selected. Forgravity thickening, however, only one thickener is selected. If the primary sludge goes to furthertreatment and/or disposal that requires high solids concentrations, a centrifuge will be used for de-watering. If anaerobic sludge digestion is the preferred option, gravity thickened sludge will be fedto the digester.

Key data are shown in Table 4. Use of sedimentation tanks requires more than 10 times the areaneeded for fine mesh sieves. The area requirement for a potential sludge dewatering centrifugehas been set at zero, since a common design is to place centrifuges on a platform above thesludge containers.

Table 4. Key data for primary treatment with conventional sedimentation tanks.

Sedimentation tanks: 2 x 250 m2 effective surface area

Gravity thickener: 22.5 m2 effective surface area

Total space requirement: ~ 575 m2


234 mg COD/L

135 mg BOD5/L


Primary sludge volume after thickening: 37.5 m3/d

Anaerobic digester volume (optional): 450 m3

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3.5. Design of processes for primary and secondary treatment

3.5.1. Alternative C

The primary treatment with Salsnes Filter fine mesh sieves will be identical to the primary treatmentin alternative A. The primary sludge will be dewatered to 25 % TS in the Salsnes Filter unit. Thenew elements in this alternative C are the proper sizing of the aeration tanks for the nitrifying acti-vated sludge, the gravity thickener for secondary sludge, and the anaerobic digester for the pri-mary-secondary sludge mixture. Two parallel trains are assumed for the activated sludge stage,while one thickener and one anaerobic digester are considered sufficient.

The key data are shown in Table 5. To supply the oxygen needed for the activated sludge aerationtanks, the blowers will draw 2260 kWh/d. The solids concentration of the mixture of primary andsecondary sludge going to the anaerobic digester is 4.4 % TS.

Table 5. Key data for primary treatment with Salsnes Filter fine mesh sieves, followedby nitrifying activated sludge.

Primary treatment equipment: 3 x Salsnes Filter model 5000/6000


216 mg COD/L

126 mg BOD5/L



Activated sludge aeration tanks: 2 x 2688 m3 volume, 1400 m2 area

Average oxygen demand: 188 kg O2/h

Average secondary sludge production: 1836 kg TS/d

Gravity thickener for secondary sludge: 73.4 m2 effective surface area

Secondary sludge volume after thickening: 91.8 m3/d

Average sludge mass to anaerobic digester: 4536 kg TS/d

Anaerobic digester volume: 1231 m3

3.5.2. Alternative D

The primary treatment with conventional sedimentation tanks will be identical to the primary treat-ment in alternative B. The new elements in this alternative D are the proper sizing of the nitrifyingactivated sludge aeration tanks, the thickener for co-thickening of both primary and secondarysludge, and the anaerobic digester for the mixture of primary and secondary sludge. Two paralleltrains are assumed for the activated sludge stage, while one thickener and one anaerobic digesterare considered sufficient.

The key data are shown in Table 6. To supply the oxygen needed for the activated sludge aerationtanks, the blowers will draw 2330 kWh/d. The solids concentration of the mixture of primary andsecondary sludge going to the anaerobic digester is 3.0 % TS.

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Table 6. Key data for primary treatment with conventional sedimentation tanks, fol-lowed by nitrifying activated sludge.

Sedimentation tanks: 2 x 250 m2 effective surface area


234 mg COD/L

135 mg BOD5/L


Activated sludge aeration tanks: 2 x 3144 m3 volume, 1640 m2 area

Average oxygen demand: 194 kg O2/h

Average secondary sludge production: 2178 kg TS/d

Gravity thickener for primary-secondary sludge: 88.6 m2 effective surface area

Primary + secondary sludge volume after thickening: 146.4 m3/d

Average sludge mass to anaerobic digester: 4428 kg TS/d

Anaerobic digester volume: 1757 m3

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4. Comparison of alternatives

4.1. Cost and area requirement

Cost figures for the Salsnes Filter fine mesh sieves are based on the year 2000 standard exportprice, plus 50 %. The additional 50 % is supposed to cover freight, other expenses and a normalprofit margin for the local distributor. All other civil works, process equipment and operation costfigures are based on the cost level in the Norwegian market in year 2000.

Costs for piping or channels between processes are excluded, since these costs are assumedequal for either of the primary treatment options. The Salsnes Filter fine mesh sieves are placedinside a shed. Other than that, the treatment plant is supposed to be outdoors, with open basins(except of course for the anaerobic digester).

An energy cost of NOK 0.40/kWh has been used, together with an operator expense of NOK 150per manhour. Maintenance costs for the Salsnes Filter fine mesh sieves are based on experiencefrom plants operating in Norway, and include all parts to be replaced every 2 to 5 years in a sched-uled maintenance program. In this comparison the cost for spare parts for the Salsnes Filter finemesh sieves has been set at 300 % of the cost in Norway, to make sure that all possible expensesassociated with operating through a distributor in a foreign country are covered. For all other proc-esses, annual maintenance costs are calculated as 1.5 % of the investment costs.

The cost for land has been set at zero. However, for a specific project where the cost of land isknown, the difference in area requirements for the different alternatives can be used to enter thosecosts into the process evaluation.

Finally, all costs in our comparison excludes tax and engineering fees.

Table 7 compares the costs for primary treatment, according to process alternatives A) and B)shown in Figure 2, for plants with an average flow of 18000 m3/d and a wastewater composition asdescribed in paragraph 3.1. For primary treatment only, the Salsnes Filter fine mesh sieves needless than 10 % of the area required for conventional sedimentation, and the investment cost isabout 50 % of the cost for conventional sedimentation. For primary treatment including thickeningand dewatering, the Salsnes Filter fine mesh sieves are also expected to save NOK 100 000 peryear in operation and maintenance (O & M) costs. For plants without thickening and dewatering,the operation and maintenance costs are lower for sedimentation than for the Salsnes Filter finemesh sieves. The reason for this is that dewatering is an integral part of the sieves, and that theenergy consumption for operation of the sieves is higher than the energy consumption necessaryfor the intermittent operation of sludge pumps and scrapers in a conventional sedimentation proc-ess.

Table 7. Comparison for plants with primary treatment only. Current exchange rate ofNOK 9.50 per USD.

Primary treatmentThickening

anddewatering

Investmentcost,

1000 NOK

*O & Mcost,

1000 NOK/year

Arearequirement,

m2

Salsnes Filter fine mesh sieves Included ---- 290 50

Not included 4140 220 540Sedimentation

Included 5790 390 575

* O & M: Operation and maintenance

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Table 8 shows the savings in investment, O & M and area requirement by using Salsnes Filter finemesh sieves, instead of conventional sedimentation, for primary treatment. For the previouslymentioned plant, with an average flow of 18000 m3/d, use of Salsnes Filter fine mesh sieves offerssubstantial savings in investment costs and required area. The differences in O & M costs aremarginal, and slightly lower for conventional sedimentation for the last two process alternatives.

Table 8. Cost savings and reductions in area requirement by using Salsnes Filter finemesh sieves, instead of conventional sedimentation, for primary treatment.Current exchange rate of NOK 9.50 per USD.

ProcessInvestment

savings,1000 NOK

O & Msavings,

1000 NOK/year

Reduced arearequirement,

m2

Primary treatment only. Sludge with high TS to further treatment and/or disposal.

---- 100 525

Primary treatment only. Sludge to anaerobic digestion

---- - 70 480

Primary treatment plus nitrifying activatedsludge. Sludge to anaerobic digestion.

---- - 20 790

4.2. Discussion

It is important to note that the comparisons in paragraph 4.1 are for outdoor plants, with open ba-sins. In Norway it is common to have treatment plants inside buildings and also have covered re-actors. This will increase both the investment and operation costs, due to more civil works and theneed for heating and ventilation. Costs are more or less proportional to the area requirement forthe processes, and will significantly increase the advantage of using the Salsnes Filter fine meshsieves. With a covered plant, use of Salsnes Filter fine mesh sieves will save from NOK 20 000 peryear to NOK 200 000 per year in O & M costs, for the process alternatives shown in Table 8. Therewill also be a significant increase in the investment cost savings compared to the numbers shown inTable 8.

Annual costs (capital cost + O & M cost) have not been calculated for the different processes, sincedifferent interest rates and years of depreciation may be used in different markets. However, withthe large savings in investment costs indicated in this feasibility study, it is obvious that use ofSalsnes Filter fine mesh sieves for primary treatment will result in significantly reduced annualcosts. This will be the case even with open basins, no cost for land and the two process alterna-tives with lower O & M costs for conventional sedimentation than for sieves.

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5. ReferencesATV (1991) Bemessung von einstufigen Belebungsanlagen ab 5000 Einwohnerwerten. RegelwerkAbwasser – Abfall. Ein Regelwerk der Abwassertechnischen Vereinigung e.V., Arbeitsblatt A 131.

Hansen, J. M. (1996) Evaluation of simple treatment methods, Phase 2: Sieves/fine screens (InNorwegian). NORVAR report no. 69-1996

Rusten, B. (2000) Salsnes Filter fine mesh sieves for primary treatment – Performance test atTiendeholmen WWTP. Aquateam report no. 00-045, October.

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