CE 321 HW 8 Solutions: Wastewater Treatment Plant Problems & Regulations, Assignments of Civil Engineering

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DEPARTMENT OF CIVIL & ENVIRONMENTAL ENGINEERING; LAFAYETTE COLLEGE

Homework Assignment 8 SOLUTIONS CE 321, Fall 2008

Problems

P1. Problem 11-5 in your text

See Example 11-2 that we discussed in class

So vol required is the max accumulated storage ~ 53 m^3 If we allow for 25% excess capacity as in Ex 11-2, then V = 52.1551.25 = 65 m*^3

P2. Problem 11-18 in your text

From class handout (or eqtn 11-29 in text) X V

Q BOD

M

F

ML

= in in

So X ( F / M )

Q BOD

V

ML

= in in = 0.438 cms * 86,400 sec/d * 150 mg/L / (2200 mg/L * 0.2 /d) = 12,900 m^3

P3. Using the following data and the equation we developed in class, determine the required reactor volume for an activated sludge plant to be built with 4 parallel reactors:

Flow rate = 10 MGD BOD in = 300 mg/L BODout = 15 mg/L MCRT = 15 days yield coeff = 0. decay coeff = 0.075 day- “mixed liquor” bacteria concentration = 2250 mg/L

**Req vol for each reactor = 10 MGD/4 *** HRT

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

+

−

=

d c

in eff ML

c k

BOD BOD

X

Y

HRT

θ

θ

1 ⎟

⎟

⎠

⎞

⎜⎜

⎝

⎛

+

=

1 0. 075 * 15

285 /

2250 /

15 0. 6 mg L mg L

d = 0.54 days

Vol = 1.35 million gal

Questions (use your text, the web, notes from our tour, and other sources to answer the following – include your sources of information)

Q1. A major reason the rotating biological contactors (RBCs) failed at the Easton Wastewater Plant in the 1980s is that the “Industrial Pretreatment Program” was not being enforced properly. Describe this program – how does a publicly-owned WWTP ensure that its processes are not negatively impacted by industrial wastes.

The Industrial Pretreatment Program (IPP) was created as part of the Clean Water Act to protect publicly- owned (i.e. by taxpayer) treatment works from the negative effects of industrial discharges, thus forcing industry to treat its own waste. Many industrial wastewaters are incompatible with biological treatment systems, which are designed to treat easily degradable organic matter. Furthermore, chemicals like heavy metals (As, Pb, Cr, Cu, Cd, etc) that are often used in industry wind up in the wastewater sludge, rendering it hazardous and unfit for disposal in municipal landfill, and unusable as biosolids ( see next question ).

The IPP regs are published in 40 CFR Part 403

The POTWs are required to identify and locate all their sewer system’s industrial uses, and develop and implement the program as part of their NPDES permit. Often some form of pretreatment is required for the industry to meet the permit requirements, which include 1) certain prohibited discharges (no fire or explosion hazard, no corrosives, no obstructions or interferences with flow, no oils, no excess heat); 2) categorical, uniform, technology-based standards based on industry classifications (for example, all electroplating companies have to meet certain discharge standards for that particular industry); 3) local limits set by the POTW based on site-specific considerations. The POTW must conduct periodic (at least annual) inspections, monitoring and record-keeping to ensure compliance with these limits. Each POTW’s IPP program is funded by user fees charged to the industries by the sewer authority.

Source: http://cfpub.epa.gov/npdes/home.cfm?program_id=

Q2. During our Easton WWTP tour, Mark Wood discussed the possibility of contracting with a company Synagro (www.synagro.com) to use treated sludge for agricultural applications. What are “biosolids”, and what are some of their beneficial uses? What regulations and concentration limits are in place to ensure that these biosolids are safe for such uses?

Biosolids are stabilized and dewatered municipal wastewater sludge. Biosolids are generally high in organic matter and nutrients and can be used as fertilizers or soil additives to enhance plant growth. They have been used in mine reclamation, agriculture, and forestry. However, they can also be high in metals and insoluble organic compounds like PCBs.

EPA regulations relating to biosolids are found in 40 CFR Part 503 – “Standards for Use or Disposal of Sewage Sludge”. The Part 503 rule contains general requirements, pollutant limits, management practices, operational standards, and requirements for frequency of monitoring, recordkeeping and reporting.

The requirements are incorporated into the POTW NPDES discharge permit. Many of them are based on human health risk assessment, using 25 representative pollutants and a variety of possible exposure pathways. There are specific limitations on concentrations of heavy metals and pathogens, as well as cumulative loading or application rates (total mass allowed per acre). Frequency of sampling depends on the amount of biosolids being applied, but is at minimum once per year. There are also requirements on how sludge can be applied, depending on potential for public contact, distance to waterbodies, and other site restrictions. There are several options for meeting the requirements, but all of them take into account both the concentrations and the management practices being used.

Source: http://www.epa.gov/owm/mtb/biosolids/

Q3. During our Easton WWTP tour, Mark Wood discussed the issue of “I&I”, which stands for inflow and infiltration. As an operator of a WWTP, why is I&I a problem?

Inflow is stormwater that is routed into the sewer system through combined sewers or other direct connections.. Under high flow conditions, much of the sewer and treatment plant capacity can be taken up with stormwater inflows into the sewer system. Infiltration is groundwater that enters cracks and leaks in the sewers when the groundwater table is above the invert (bottom) of the sewer line. These I & I sources are relatively clean water (low BOD and solids) that combine with sewage and must therefore be treated at the plant. As an operator, you want to reduce and eliminate I&I from the system so that you don’t waste capacity, energy, and ultimately dollars treating essentially clean water.