16-Civ-A3 Elementary Environmental Engineering · December 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exams, December 2016 — 98-Civ-A3 Environmental Engineering. Three hours; closed book with one candidate-prepared 8 × 11 double-sided aid sheet; approved Casio or Sharp calculator only. Seven problems are printed, each worth 20 marks, and any five constitute a complete paper (maximum 100 marks). All seven are solved here, because the set is intended as a study resource rather than an exam script. Section marks are shown in brackets at the left margin of each question and are reproduced below.
Reference texts.
Check: The page-1 marking scheme on this paper is not reliable as printed, but the mark figures printed in the left margin of each question page are internally consistent — every problem's sub-part marks sum to exactly 20, and the parts of the scheme that are given agree with them. The margin figures are adopted throughout: Q1 (6, 7, 7); Q2 (9, 6, 5); Q3 (7, 7, 6); Q4 (10, 10); Q5 (10, 10); Q6 (10, 10); Q7 (5, 6, 3, 3, 3).
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Benefit 1 — a reliable, drought-proof water supply that frees higher-quality water for other uses. Agriculture accounts for roughly 70 % of global freshwater withdrawal, so substituting treated effluent for river or groundwater abstraction on irrigated land releases a large volume of potable-quality water for municipal supply and for environmental flows. The substitution is particularly valuable because the supply is counter-cyclical: municipal effluent volume is essentially constant and is largest in dry periods when demand is highest and rivers are lowest, whereas the surface and groundwater sources it replaces are least reliable at exactly that time. Reuse also reduces the effluent load discharged to the receiving water, so the same measure that secures irrigation supply also relieves the river of nutrients and organic load.
Benefit 2 — the nutrient content of the effluent displaces purchased fertiliser. Secondary effluent typically carries 20 to 40 mg/L of nitrogen and 4 to 10 mg/L of phosphorus, together with potassium and micronutrients. At a seasonal application of some 5,000 m3 per hectare this delivers on the order of 100 to 200 kg of nitrogen and 20 to 50 kg of phosphorus per hectare per year — a substantial fraction of the crop requirement for many field crops. The farmer's fertiliser cost falls, the energy-intensive Haber-Bosch nitrogen and mined phosphate they displace are avoided, and nutrients that would have been a pollutant in the river become a resource on the land. This is the clearest practical case of the circular-economy principle in water management.
Challenge 1 — pathogen exposure and the public-health risk management it demands. Municipal effluent contains enteric bacteria, viruses and helminth ova, and the exposed populations are farm workers, nearby residents exposed to aerosols from sprinklers, and consumers of crops eaten raw. Managing this drives the whole design: the required treatment level is set by the crop and the irrigation method, following the WHO guidelines and the CCME Canada-wide Guidelines for Domestic Reclaimed Water, so that unrestricted irrigation of salad crops demands filtered, disinfected effluent meeting a stringent coliform limit, while restricted irrigation of fodder, fibre or orchard crops can accept less. It also requires the multiple-barrier approach in the field — subsurface drip rather than spray to eliminate aerosols and foliar contact, withholding periods before harvest, worker protective equipment and hygiene, crop restriction by by-law, and monitoring — plus separate purple-pipe distribution with cross-connection control and backflow prevention to keep reclaimed water out of the potable system.
Challenge 2 — salinity, sodium and accumulating trace contaminants degrading the soil over time. Municipal use adds roughly 200 to 400 mg/L of dissolved solids to the water supplied, so effluent is saltier than the source water, and because irrigation water evaporates while its salt does not, salts concentrate in the root zone over successive seasons. A high sodium adsorption ratio disperses clay particles and destroys soil structure and infiltration capacity, and boron and chloride are directly phytotoxic to sensitive crops. Long-term application also accumulates heavy metals and, increasingly a regulatory concern, persistent organic contaminants such as pharmaceuticals and per- and polyfluoroalkyl substances in the soil. Management therefore requires monitoring of the electrical conductivity and sodium adsorption ratio of both water and soil, an added leaching fraction in the irrigation schedule to flush salts below the root zone — which in turn requires adequate drainage or the salt simply returns with a rising water table — gypsum amendment to correct sodicity, and crop selection matched to the salt tolerance achievable. There is a further practical challenge in the mismatch of timing: effluent is produced year-round while irrigation demand is seasonal, so storage of a whole winter's production, or an alternative discharge, must be provided.
Colour — enhanced coagulation with alum or ferric chloride, followed by flocculation, sedimentation and filtration. True colour in a surface water is caused by dissolved natural organic matter, principally humic and fulvic acids leached from soils and vegetation, which carry a negative charge at natural pH. Adding a trivalent metal coagulant — aluminium sulfate at 30 to 60 mg/L, or ferric chloride — and depressing the pH to about 5.5 to 6.0 neutralises that charge and precipitates the organic matter with the metal hydroxide floc, which is then aggregated by gentle flocculation and removed by settling and filtration. The term enhanced coagulation refers precisely to this use of a higher dose and a lower pH than turbidity removal alone would require, in order to maximise removal of dissolved organic carbon. Typical colour removal is 80 to 95 %. The step carries a second and equally important benefit: the natural organic matter removed is the principal precursor of trihalomethanes and haloacetic acids, so removing colour before chlorination is the standard means of controlling disinfection by-products.
Unpleasant odours — oxidation with ozone, or adsorption onto granular activated carbon. Taste and odour in surface waters come chiefly from geosmin and 2-methylisoborneol released by cyanobacteria and actinomycetes, compounds detectable by the human nose at only 5 to 10 ng/L. Ozone at 1 to 3 mg/L with 5 to 15 minutes of contact attacks these compounds both directly and through the hydroxyl radicals it generates, destroying them rather than transferring them elsewhere, and it simultaneously oxidises iron, manganese and sulfide and provides powerful primary disinfection. Its limitation is that partial oxidation of natural organic matter produces assimilable organic carbon that can support biological regrowth in the distribution system, so ozone is normally followed by biologically active granular activated carbon, which both adsorbs residual odour compounds and biodegrades the assimilable carbon. Where ozone is not justified, granular activated carbon alone, or powdered activated carbon dosed seasonally during algal blooms, is the standard alternative. Aeration is effective only for volatile odours such as hydrogen sulfide and does little for geosmin, which is why it is not the general answer.
Pathogens — ultraviolet disinfection, in a multiple-barrier arrangement with filtration and a chlorine residual. Ultraviolet light at 254 nm is absorbed by nucleic acids and forms pyrimidine dimers that prevent replication, inactivating the organism. Its decisive advantage for a surface water source is its effectiveness against Cryptosporidium and Giardia: a dose of about 40 mJ/cm2 delivers 3-log or greater inactivation of these protozoa, which are highly resistant to chlorine and were responsible for the Milwaukee and North Battleford outbreaks. Ultraviolet treatment also forms no halogenated by-products and requires only seconds of contact. It provides no residual, however, so it must be paired with a chlorine or chloramine residual to protect the distribution system against regrowth and intrusion, and it demands a low-turbidity feed because particles shield organisms from the light — which is why it belongs after filtration in the train. This arrangement is the standard Canadian multiple-barrier approach: source protection, coagulation and filtration for physical removal, ultraviolet for the chlorine-resistant protozoa, and a chlorine residual for bacteria, viruses and distribution protection.
| Basis of comparison | Anaerobic treatment | Aerobic treatment |
|---|---|---|
| 1. Terminal electron acceptor and energy balance | Oxygen is absent; organic matter, sulfate or carbon dioxide serves as the electron acceptor. No aeration is required, so energy consumption is minimal and the process is a net energy producer through the methane it yields — roughly 0.35 m3 of methane per kg of COD removed. | Dissolved oxygen is the electron acceptor and must be supplied continuously by blowers or surface aerators. Aeration typically accounts for 45 to 60 % of the entire plant's electricity demand, so the process is a substantial net energy consumer. |
| 2. Sludge production and end products | Cell yield is low, about 0.05 to 0.1 kg VSS per kg COD removed, because so little energy is captured for growth, so sludge production is five to ten times smaller and disposal costs correspondingly less. Products are methane, carbon dioxide, hydrogen sulfide and ammonia. | Cell yield is high, about 0.4 to 0.6 kg VSS per kg BOD removed, so large volumes of waste activated sludge are generated and must be thickened, digested and disposed of. Products are carbon dioxide, water, nitrate and new biomass. |
| 3. Kinetics, effluent quality and application | Growth is slow, requiring long solids retention times of 15 to 30 days and temperatures near 35 °C for mesophilic operation, and the process is sensitive to shock loads, pH and toxicity. Effluent quality is only moderate and normally needs aerobic polishing. Best suited to high-strength wastes above about 1,500 mg/L COD, and to sludge stabilisation. | Growth is rapid, with solids retention times of 5 to 15 days at ambient temperature, and the process is robust and quick to recover. It produces a high-quality, low-BOD, nitrified effluent suitable for direct discharge. Best suited to dilute wastes such as municipal sewage. |
| Basis of comparison | Extended aeration | Conventional activated sludge |
|---|---|---|
| 1. Retention times and loading | Long hydraulic retention time of 18 to 36 hours and a solids retention time of 20 to 30 days or more, at a low food-to-microorganism ratio of about 0.05 to 0.15 kg BOD per kg MLSS per day. The aeration basin is therefore large for the flow treated. | Hydraulic retention time of 4 to 8 hours and solids retention time of 5 to 15 days, at a food-to-microorganism ratio of about 0.2 to 0.5. The basin is compact, which matters where land is expensive. |
| 2. Sludge yield and stabilisation | The long sludge age forces the biomass into endogenous respiration, in which the organisms consume their own cell material. Waste sludge is minimal and already aerobically stabilised, so no separate anaerobic digester is normally required — a decisive simplification for a small plant. | Sludge is produced in quantity and is not stabilised, so separate thickening and anaerobic or aerobic digestion is required, with the compensating advantage that the digester generates biogas that can be recovered as energy. |
| 3. Energy, robustness and typical application | Energy per unit of BOD removed is high because aeration continues through the long endogenous phase, but the large volume dilutes shock loads and gives excellent resistance to flow and load variation, and the long sludge age reliably supports nitrification. Typical of package plants, small rural communities, camps and institutions, commonly as an oxidation ditch or SBR. | Energy per unit of BOD removed is lower and the process is more efficient at scale, but it demands closer operator control of return and waste sludge rates and is more vulnerable to upset and to sludge bulking. Typical of medium and large municipal plants where skilled operating staff are on site. |
| Basis of comparison | Primary treatment | Secondary treatment |
|---|---|---|
| 1. Mechanism | Purely physical — gravity sedimentation of settleable solids and flotation of grease and scum in a quiescent clarifier, following screening and grit removal. No biological or chemical conversion occurs. | Biological — a managed microbial population metabolises dissolved and colloidal organic matter, converting it to carbon dioxide, water and new cell mass which is then separated by settling. Suspended-growth (activated sludge) or attached-growth (trickling filter, RBC, MBBR) configurations. |
| 2. Target contaminants and performance | Removes only settleable and floatable material: 50 to 70 % of suspended solids and 25 to 40 % of BOD. Dissolved and colloidal organics pass through essentially untouched, as do nutrients and pathogens. | Removes the dissolved and colloidal organic fraction that primary treatment cannot: 85 to 95 % of BOD and of suspended solids overall, giving an effluent typically below 25 mg/L of each, and with a long sludge age it also nitrifies ammonia. Biological nutrient removal extends it to nitrogen and phosphorus. |
| 3. Cost, complexity and regulatory standing | Simple, passive and cheap — low capital cost, essentially no energy beyond sludge pumping, minimal operator attention. Produces raw primary sludge requiring stabilisation. Primary treatment alone no longer satisfies Canadian requirements. | Energy-intensive, mechanically complex and requiring skilled continuous operation, at several times the capital and operating cost. Produces waste activated sludge. Secondary treatment is the minimum national standard under the Wastewater Systems Effluent Regulations, which set 25 mg/L CBOD and 25 mg/L suspended solids. |
