Environmental Engineering Reference
In-Depth Information
BRA releases 97 kg NO
3
-
-N with the effluent, at an average concentration of 43 mg/l. At an
assumed methanol:NO
3
-
-N ratio of 3.2:1 required by denitrification, 310.4 kg methanol per
day will be needed. At a methanol density of about 0.8 kg/l, 388 L of methanol will be
needed. Industrial grade methanol has a 98% concentration, which results in a total
requirement of 396 L. At a price of $0.50/L, the cost of methanol will be $198/day.
Jar tests showed that treatment with an average of 50 mg FeCl
3
/l wastewater, followed by
sand filtration, could reduce the TSS in the treated effluent well below 10 mg/l. At this dose,
113 kg FeCl
3
/day would be needed. At an average market price of $300/metric ton FeCl
3
,
the daily cost should be $34.
The operation of a full-scale wastewater treatment station would require full-time labor,
including weekends. This will add up to 56 hrs of labor per week, or 240 hrs per month. At
an estimated wage of $10 per hour, labor costs will add $2,400 per month to costs.
Operation of the wastewater treatment system would minimize certain operations costs
currently faced by BRA. Water temperature did not decrease by more than 1-1.5°C as water
passed through the pilot station during the 3-4 hours of treatment. However, the pilot
station was operated during the summer, and high environmental temperature could have
affected the rate of temperature loss. Considering that the treatment station also would
function during the winter and that water would be held in storage tanks before and after
treatment, we assumed that temperature would decrease by as much as 7°C. Because BRA
spends between $10,000 and $28,000 per month (depending on season) on fuel oil to heat
water, 75% heat recovery would represent $14,250 per month in savings, from the current
average of $19,000. That is, only $4,750/mo ($158.33/day) would be spent to bring
replacement water to culture temperature, and $14,250 per month would represent savings.
Releasing the waste stream instead of treating and reusing it adds to BRA's operating costs as
wastewater discharge bill from the city of $14,000/month ($467/day). The energy to pump
replacement water from the wells adds $450/month ($15/day) to operations costs. The
summation of these costs results in total estimated expenses of $595.83/day ($17,875/month)
for consumable materials. Against this total, $33,450 is the actual average cost of heating the
replacement water plus the municipal water treatment charge and the energy cost for
pumping the water from the wells. Were the entire effluent reused, operations costs of
wastewater treatment at BRA would be reduced by $15,575 per month. Hence, much of the
economic determination of whether to go forward with wastewater treatment and reuse will
depend upon construction costs and the amortization into ongoing payments.
Our findings regarding costs and savings for treatment of effluent were such that BRA has
gone forward with investment in full-scale effluent treatment. While the treatability of
aquaculture effluent and the costs structure for treatment are specific to a given operation,
the approach we took for assessing treatability and for designing and evaluating a treatment
train are general, and will have relevance to a range of recirculating aquaculture system
operations.
4. Conclusion
Recirculating aquaculture systems have been developed to produce high-value species for
year-round supply of markets, to free production from site-related constraints, to minimize
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