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On-site Wastewater Treatment Systems – Technical issues UNESCO-IHE, MUI Department, Elisabeth v. Münch, PhD On-site wastewater treatment systems - a brief overview of technical issues Elisabeth v. Münch, PhD Department of Municipal Infrastructure, UNESCO-IHE, Westvest 7, 2611 AX Delft, The Netherlands February 2005 Page 2 On-site Wastewater Treatment Systems – Technical issues UNESCO-IHE, MUI Department, Elisabeth v. Münch, PhD TABLE OF CONTENTS 1. 2. 3. Definition of Terms _______________
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  On-site Wastewater Treatment Systems – Technical issuesUNESCO-IHE, MUI Department, Elisabeth v. Münch, PhD   On-site wastewater treatment systems - a brief overview of technical issues Elisabeth v. Münch, PhD Department of Municipal Infrastructure, UNESCO-IHE, Westvest 7, 2611 AX Delft, TheNetherlandsFebruary 2005 Page 2  On-site Wastewater Treatment Systems – Technical issuesUNESCO-IHE, MUI Department, Elisabeth v. Münch, PhD   TABLE OF CONTENTS 1. Definition of Terms _____________________________________________________4    2. Evaluation and Classification of Disposal Sites _______________________________5    3. Processes for Decentralised Wastewater Treatment Systems_____________________6    3.1.Anaerobic Treatment Processes_______________________________________6  3.1.1. Septic Tanks___________________________________________________63.1.2. Upflow Anaerobic Sludge Blanket Reactor (UASB)____________________7 3.2.Ponds____________________________________________________________7   3.3.Upgrading Pre-Treated Wastewater___________________________________8  3.3.1. Intermittent Sand Filter___________________________________________83.3.2. Rock Filters____________________________________________________8 3.4.Aerobic Suspended Growth Systems___________________________________9  3.4.1. Sequencing Batch Reactor (SBR)___________________________________93.4.2. Extended Aeration_______________________________________________93.4.3. Membrane Bioreactor (MBR)_____________________________________10 3.5.Aerobic Fixed Film Systems_________________________________________11  3.5.1. Rotating Biological Contractor (RBC)______________________________113.5.2. Submerged Aerated Filter (SAF)__________________________________113.5.3. Moving Bed Bioreactor (MBBR)__________________________________11 3.6.Constructed Wetlands______________________________________________12   3.7.Other Issues______________________________________________________12  3.7.1. Nutrient Removal______________________________________________123.7.2. Sludge Management____________________________________________133.7.3. Odour________________________________________________________13 3.8.Comparison of Processes___________________________________________13    4. Final Disposal Methods_________________________________________________16    4.1.Final Disposal Methods for Normal Site Conditions_____________________16   4.2.Final Disposal Methods for Difficult Site Conditions____________________16   4.3.Final Disposal Methods for Adverse Site Conditions____________________16    5. Wastewater Collection, Treatment and Disposal for Sewered Areas______________17    5.1.Collection Systems_________________________________________________17   5.2.Treatment Systems________________________________________________17  5.2.1. Pre-engineered Package Plants____________________________________175.2.2. Individually Designed and Constructed Systems______________________18 5.3.Disposal and Reuse Systems_________________________________________18   6. References____________________________________________________________18   Page 3  On-site Wastewater Treatment Systems – Technical issuesUNESCO-IHE, MUI Department, Elisabeth v. Münch, PhD   1. DEFINITION OF TERMSOn-site Treatment The term on-site treatment is usually used to refer to “small” wastewater treatment plants withlocal effluent disposal (most often subsurface disposal). It usually involves a very basicsewerage system to convey wastewater to the treatment process. The treated effluent pipes areusually very short, because the aim is to locate the treatment process and the disposal site inclose proximity to each other. Small Treatment Plants The International Water Association Specialist Group on Small WWTP has defined a smallwastewater treatment plant (WWTP) as being one for less than 2000 person equivalents (pe)or a daily flow of 200 m 3  /d. In comparison, the Norwegian classification system calls atreatment plant “small” if it treats the effluent of 35-500 persons. Indeed, there is no clear-cutdefinition of what size exactly is “small” for WWTPs.WWTPs that have the capacity to treat effluent from < 35 persons (i.e. scattered dwellings of < 7 houses) are called mini-treatment plants (or on-site plants) in Norway. These plants areonly used in cases where soil infiltration cannot be used due to impermeable soils.Mini-treatment plants normally treat all the wastewater from the house (grey as well as black water), and they normally use pre-treatment in a septic tank followed by a unit based onbiological or chemical processes or combinations of the two. Decentralised Wastewater Management There is no generally accepted definition of what “decentralised” wastewater managementmeans. It is usually used to imply something “small” and “compact”.Decentralised wastewater management employs collection, treatment, and disposal/reuse of wastewater from individual homes, clusters of homes, isolated communities, industries orinstitutional facilities, as well as from portions of existing communities at or near the point of waste generation. Decentralised systems maintain both the solid and liquid fractions of thewastewater near their point of srcin, although the liquid portion and any residual solids canbe transported to a centralised point for further treatment and reuse (Crites andTchobanoglous, 1998).Decentralised WWTPs will normally (but not always) receive a more concentratedwastewater than centralised plants, and flow and variations in composition are greater.Generally speaking, the smaller the system, the larger the variations in flow and compositionwill be. Other Wastewater Treatment Terms The conditions present in a biological treatment system are commonly classed into thefollowing three categories: Page 4  On-site Wastewater Treatment Systems – Technical issuesUNESCO-IHE, MUI Department, Elisabeth v. Münch, PhD    An aerobic state is characterised by the presence of oxygen (may or may not containnitrate).  An anaerobic state is characterised by the absence of oxygen or nitrate.  An anoxic state is characterised by the absence of oxygen, but presence of nitrate.This latter distinction between anaerobic and anoxic is only important for discussingbiological nutrient removal concepts.Person equivalent or p.e. (the amount of wastewater discharged per person; usually taken tomean 200 L/d/person and 60 g BOD/d/person).BOD: BOD stands for “biological oxygen demand” and is a measure for the strength of thewastewater with regard to organic matter. It is expressed as mg/l or kg/d.Effluent: The water flow leaving a wastewater treatment system. 2. EVALUATION AND CLASSIFICATION OF DISPOSAL SITES Table 1below summarises the characteristics of so-called “normal”, “difficult” and “adverse”site conditions. The type of site condition is an important factor in choosing the effluentdisposal method. Table 1. Classification of site conditions (after Qasim, 1999)   Normal siteconditions   Difficult siteconditions   Adverse siteconditions Suitable forpercolationSubsoil type Low permeability ImperviousWater table depth Deep High Very highBedrock Unfractured High and fractured High and fracturedGround surface slope Favourable Unfavourable SteepFlooding None Occasional FrequentDistance from watersupply wells,buildings,escarpmentsFar away Quite close CloseLot size Large Small SmallDisposal methods forseptic tank effluent(see also SectionGravity flow over aconventionalpercolation trench orbed.Further treatmentneeded (aerobicprocesses orintermittent sandfilters). Periodicdosing of a disposalfield by pump ordosing syphon.Drastic change inwater conservation,treatment andrecycling.3) Page 5
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