ebook ipal grit chamber.pdf

Desain Instalasi Pengolah Air Limbah (IPAL)

TatyAlfiah/Sabtu, 19 Oktober 2013Hlm - 1

Mata Kulia Desain Instalasi Pengolah Air Limbah (IPAL)Kode MK / Sks / Smtr 1309701 / 4 sks / VIIProgram Studi Teknik Lingkungan

Fakultas Teknik Sipil dan PerencanaanInstitut Teknologi Adhi Tama Surabaya [ITATS}

Dosen Taty Alfiah, ST.MT.

Grit Chamber



Pengertian dan Fungsi :

Grit :Fraksi organik berat dari bahan padat air limbah dengan kepadatan relatifkira-kira 2,5 sehingga punya kecepatan mengendap yang jauh lebih besardaripada bahan padat air limbah organik (kira-kira 30 mm/det) dibandingkandengan kecepatan mengendap bahan padat air limbah organik yaitu sebesar3 mm/det

Example : Grit consists of: sand, gravel, pebbles, silt, broken glass, cinders (ash),heavy particulate matter (e.g., corn kernels, bone chips, coffeegrounds, egg shells, seeds)

Karakteristik grit :

Tidak membusuk , anorganik Mampu mengendap sec. gravitasi BJ grit =2,65 > BJ organik = 1,02 SG range = 1,3 2,7 tipikal 2,65, atau 1,5 - 2,7 [Qasim] kadar air 13-65%, kadar volatil 1-56%, diamtr >= 0,2 mm Pasir, biji-2an, potongan tulang, kulit telur, dll

Most grit collected at WTTPs - nearly 90 percent on average - is larger than 70mesh in size. In some coastal areas, smaller grit particles down to 100 mesh canbe more common.

Grit Size Comparison Chart1

Sieve Mesh # Inches Microns Similarly-Sized Material

14 .0555 1400 -

28 .0280 700 Beach Sand

60 .0098 250 Fine Sand

100 .0059 150 -

200 .0029 74 Portland Cement



Grit Chamber bertujuan untuk : (i) protect mechanical equipments from abrasion. Melindungi atau

mencegah terjadinya gesekan pada peralatan mekanik dan pompa akibatadanya pemakaian yang tidak perlu dan akibat adanya abrasi

(ii) reduce formation of heavy deposits in pipelines or channels. Mencegahterjadinya penyumbatan pada pipa akibat adanya endapan kasar di dalamsaluran

Mencegah timbulnya efek penyemenan di dasar sludge digester danprimary sedimentation tank

Menurunkan akumulasi material inert di dalam kolam aerasi dan sludgedigester yang akan mengurangi volume yang dapat digunakan.

(iii) reduce the frequency of tank/reactor cleaning caused by excessiveaccumulations of grits.

Fungsi : Grit removal is aimed on removing sand, gravel or other heavy solid

materials that have subsiding velocities or specific gravities greater thanthose of organics in wastewater.

Letak dalam IPAL/WWTP : Grit chambers are commonly located after screening and before primary

sedimentation tanks.

Grit chambers are of two types: mechanically cleaned and manually cleaned. In mechanically cleaned grit chamber, scraper blades collect the grit

settled on the floor of the grit chamber. The grit so collected is elevated tothe ground level by several mechanisms such as bucket elevators, jet pumpand air lift. The grit washing mechanisms are also of several designs mostof which are agitation devices using either water or air to produce washingaction.

Manually cleaned grit chambers should be cleaned atleast once a week.The simplest method of cleaning is by means of shovel.

DesignKec. Aliran

Tipikal = 0,15 0,3 m/det, terbaik 0,225 m/det 0,2 mm, sg 2,65

Kecepatan mengendap Hk Newton :

3C)d4g(Vs

D

s



Kecepatan settling Hk Stokes :

2s ).d.(.18

Vs g

dimana :vs = kec. Settlingps = densitas partikelp = densitas fluidau = viscositas fluidag = perc. Gravitasid = diameter partikel

Kecepatan scouring :

f1).g.d-.(s8.Vs

dimana :Vs = kec. HorisontalS = spesifik graviti partikelG = perc. GravitasiD = diameter partikel = konstanta = 0,04 0,06f = faktor friksi darcy-Weisbach = 0,02 0,03

Secara umum ada 3 jenis tipe grit chamber, yaitu:

1. Horizontal flow grit chamber2. Aerated grit chamber3. Vortex Grit Chamber



Horizontal-Flow Grit Chamber

The oldest type of grit chamber for grit removal. The systems are based on ahorizontal velocity of the flow equal to 0,3 m/s.

Arah alirannya horisontal V selalu konstan untuk tiap level debit Regulator debit : proportional weir, parshal flume, parabola flume

The horizontal flow grit chamber is the oldest type of grit removal system.Grit is removed by maintaining a constant upstream velocity of 0.3 m/s (1 ft/s).Velocity is controlled by proportional weirs or rectangular control sections, such asParshall flumes. In this system, heavier grit particles settle to the bottom of thechannel, while lighter organic particles remain suspended or are resuspended andtransported out of the channel.



Aerated Grit Chamber

Menggunakan proses aerasi dengan aliran spiral dan menggunakan diffusercompressor udara

In aerated grit chambers, air is introduced along one side of a rectangular tank tocreate a spiral flow pattern perpendicular to the flow through the tank. Air flowhave to be adjusted in any specific installation:

if air flow creates a too high velocity of roll (or agitation), grit will be carriedout of the chamber

if air flow creates a too small velocity or roll, organic material will beremoved with the grit. With proper adjustment, almost 100% of grit will beremoved and the grit obtained will be well whashed (no content of organicmatter).

Some advantages of aerated grit chambers include: Consistent removal efficiency over a wide flow range. A relatively low putrescible organic content may be removed with a well

controlled rate of aeration. Performance of downstream units may be improved by using pre-aeration

to reduce septic conditions in incoming wastewater. Aerated grit chambers are versatile, allowing for chemical addition, mixing,

preaeration, and flocculation.

























Design considerations for aerated grit chambers include the following (WEF 1998): Air rates typically range from 0.3 to 0.7 m3/mmin (3 to 8 ft3/ft.min) of

tank length. A typical minimum hydraulic detention time at maximum instantaneous

flow is two minutes. Typical length-to-width ratio is 2.5:1 to 5:1. Tank inlet and outlet are positioned so the flow is perpendicular to the

spiral roll pattern. Baffles are used to dissipate energy and minimize short circuiting.















Vortex-Type Grit Chamber

In vortex-type grit chambers, a vortex flow pattern is used to remove heavyparticles.The vortex flow patter can be created by using a rotating turbine or by entering theflow tangentially at the top of the unitBerupa tangki silinder di mana air limbah yang masuk akan menghasilkan polaaliran vortex, gaya sentrifugal dan gravitasi sehingga tejadi pemisahan grit



Vortex-Type Grit Chamber



CONTOH PERENCANAAN : HORIZONTAL FLOW GRIT CHAMBER

DIRENCANAKAN :



Catatan :Untuk desain kedalaman minimum grit storage=10 cm, maka untuk perencanaanini dipakai kedalaman grit storage sebesar 10 cm namun yang terisi grit hanyasebesar 1 cm

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